Remove obsolete config

See merge request !56

There should not be any Servlet container around which does not support
JSON by default. The backend does not have an AppCache manifest so a
mapping is not needed here either.

Since error handlers use JSON for responses there is no need for HTML
error pages anymore.

Add multi-version API support via media types

See merge request !55

API v3:
* Media type: application/vnd.de.thm.arsnova.v3+json
* Serializes date as strings conforming to ISO 8601.

API v2:
* Media type: application/vnd.de.thm.arsnova.v2+json
* Serializes dates as timestamps including nanoseconds.

An indentation setting has been added: `api.indent-response-body:false`.

Exception logging

See merge request !54

Add messages to exceptions and slightly refactor exception handling.

Exceptions are now always passed to the logger. The logger decides based
on configuration whether and how they are logged. Logging messages were
adjusted for consistency.

Improve exception handling

See merge request !52

Execption messages in API responses is disabled by default because they
can contain sensitive data. The config property
`api.expose-exception-messages` has been added and is described in the
developer documentation.

Error responses now expose exception details.

Exeption handling was broken by
56fe085e (GH-27).

Migrate remaining Spring XML config to Java

See merge request !51

Remove redundant NovaView class

See merge request !50

All former features of NovaView have been merged into CouchDB4J so it
is no longer needed.

The following replacements have been made:
* New design document name
    - New view name: (conditions) keys => values
        * Full old view name: (conditions) keys => values

* user/
    - user/by_creation_for_inactive: (activationKey) creation => _rev
        * user/inactive_by_creation: (activationKey) creation => _rev
    - doc_by_username: username => doc
        * user/all: username => doc

The following replacements have been made:
* New design document name
    - New view name: (conditions) keys => values
        * Full old view name: (conditions) keys => values

* comment/
    - by_sessionid: sessionId => _rev
        * interposed_question/count_by_session: sessionId => doc
    - by_sessionid_creator_read: sessionId, creator, read => _rev
        * interposed_question/count_by_session_reading_for_creator, sessionId, creator, 'read' | 'unread' => doc
    - by_sessionid_read: sessionId, read => _rev
        * interposed_question/count_by_session_reading: sessionId, 'read' | 'unread' => doc
    - doc_by_sessionid_creator_timestamp: sessionId, creator, timestamp => doc
        * interposed_question/by_session_and_creator: sessionId, creator, timestamp => doc
    - doc_by_sessionid_timestamp: sessionId, timestamp => doc
        * interposed_question/by_session_full: sessionId, timestamp => doc
        * interposed_question/by_session: sessionId => subject, timestamp, _rev, read, creator

The following replacements have been made:
* New design document name
    - New view name: (conditions) keys => values
        * Full old view name: (conditions) keys => values

* answer/
    - by_questionid: questionId => _rev
        * answer/cleanup: questionId => _id
    - by_questionid_piround_text_subject: questionId, piRound, abstention, answerText, answerSubject, successfulFreeTextAnswer => _rev
        * skill_question/count_all_answers_by_question: questionId, answerText, answerSubject, successfulFreeTextAnswer => 1
        * skill_question/count_answers_by_question_and_piround: questionId, piRount, answerText => 1
        * skill_question/count_abstention_answers_by_question: (abstention) questionId => doc
        * skill_question/count_total_answers_by_question: questionId => doc
        * skill_question/count_total_answers_by_question_and_piround: questionId, piRount => 1
    - by_sessionid: sessionId => _rev
        * skill_question/count_answers_by_session: sessionId => doc
    - by_sessionid_variant: sessionId, variant => _rev
        * skill_question/count_answers_by_session_and_question_variant: sessionId, questionVariant => doc
    - by_user_sessionid: user, sessionId => _rev
        * answer/by_user: user, sessionId => questionId
    - doc_by_questionid_timestamp: questionId, timestamp => doc
        * skill_question/freetext_answers_full: questionId, timestamp => doc
    - doc_by_questionid_user_piround: questionId, user, piRound => doc
        * answer/by_question_and_user_and_piround: questionId, user, piRound => doc
    - doc_by_user_sessionid: user, sessionId => doc
        * answer/by_user_and_session_full: user, sessionId => doc
    - questionid_by_user_sessionid_variant: user, sessionId, questionVariant => questionId
        * answer/by_user: user, sessionId => questionId
        * answer/variant_by_user: user, sessionId, questionVariant => questionId
    - questionid_piround_by_user_sessionid_variant: user, sessionId, questionVariant => questionId, piRound
        * answer/variant_by_user_and_piround: user, sessionId, questionVariant => questionId, piRound

The following replacements have been made:
* New design document name
    - New view name: (conditions) keys => values
        * Full old view name: (conditions) keys => values

* content/
    - by_sessionid: sessionId => _rev
        * skill_question/count_by_session: sessionId => doc
    - by_sessionid_variant_active: sessionId, questionVariant, active, subject, text.substr() => _rev
        * skill_question/by_session_only_id_for_all: (active == 1) sessionId => null
        * skill_question/flashcard_count_by_session: (questionVariant == "flashcard") sessionId => doc
        * skill_question/lecture_question_count_by_session: (questionVariant == 'lecture') sessionId => doc
        * skill_question/lecture_question_ids_by_session_and_subject: (questionVariant == 'lecture') sessionId, subject => _id
        * skill_question/lecture_question_ids_by_session_for_all: (questionVariant == "flashcard" && active == 1) doc.sessionId, doc.subject, doc.text => {}
        * skill_question/lecture_question_subjects_by_session: (questionVariant == 'lecture') sessionId => subject
        * skill_question/preparation_question_count_by_session: (questionVariant == 'preparation') sessionId => doc
        * skill_question/preparation_question_ids_by_session_and_subject: (questionVariant == 'preparation') sessionId, subject => _id
        * skill_question/preparation_question_ids_by_session_for_all: (questionVariant == 'preparation' && active == 1) sessionId, subject, text => {}
        * skill_question/preparation_question_subjects_by_session (questionVariant == "preparation") sessionId => subject
    - doc_by_sessionid_variant_active: sessionId, questionVariant, active, subject, text.substr() => doc
        * skill_question/by_session: sessionId, subject => doc
        * skill_question/by_session_for_all_full: (active == 1) sessionId, subject, text => doc
        * skill_question/by_session_sorted_by_subject_and_text: sessionID, subject, text => doc
        * skill_question/flashcard_by_session: (questionVariant == 'flashcard') sessionId, subject, text => doc
        * skill_question/flashcard_by_session_for_all: (questionVariant == "flashcard" && active == 1) sessionId, subject, text => doc
        * skill_question/lecture_question_by_session: (questionVariant == 'lecture') sessionId, subject, text => doc
        * skill_question/lecture_question_by_session_for_all: (questionVariant == 'lecture' && active == 1) sessionId, subject, text => doc
        * skill_question/preparation_question_by_session: (questionVariant == 'preparation') sessionId, subject, text => doc
        * skill_question/preparation_question_by_session_for_all: (questionVariant == 'preparation' && doc.active == 1) sessionId, subject, text => doc

The following views have been removed:
* skill_question/questions_by_ids: _id => doc (use all_docs)

The following replacements have been made:
* New design document name
    - New view name: (conditions) keys => values
        * Full old view name: (conditions) keys => values

* motd/
    - by_motdkey: motdkey => _rev
        * motd/by_keyword: motdkey => doc
    - doc_by_audience_for_global: (audience != 'session') audience => doc
        * motd/admin: (audience != 'session') 1 => doc
        * motd/for_tutors: (audience == 'tutors' || audience == 'loggedIn') 1 => doc
            - 2 backend requests
        * motd/for_all: (audience == 'all') 1 => doc
        * motd/for_loggedin: (audience == 'loggedIn') 1 => doc
        * motd/for_students: (audience == 'students' || audience == 'loggedIn') 1 => doc
            - 2 backend requests
    - doc_by_sessionkey: (audience == 'session') sessionkey => doc
        * motd/by_sessionkey: (audience == 'session') sessionkey => doc
* motdlist/
    - doc_by_username: username => doc
        * motd/list_by_username: username => doc

The following views have been removed:
* motd/all: 1 => doc (never used)

The following replacements have been made:
* New design document name
    - New view name: (conditions) keys => values
        * Full old view name: (conditions) keys => values

* session/
    - by_courseid: (courseId && sessionType != 'public_pool') courseId => _rev (include_docs)
        * session/by_courseid: (courseId && sessionType != 'public_pool') courseId => doc
    - by_keyword: keyword => _rev (include_docs)
        * session/by_keyword => doc
    - by_lastactivity_for_guests: (sessionType != 'public_pool' && creator.indexOf('Guest') == 0) lastOwnerActivity || creationTime => _rev
        * session/by_last_activity_for_guests: (sessionType != 'public_pool' && creator.indexOf('Guest') == 0) lastOwnerActivity || creationTime => _rev
    - partial_by_ppsubject_name_for_publicpool: (sessionType == 'public_pool') ppSubject, name => partial doc
        * session/public_pool_by_subject: (sessionType == 'public_pool') ppSubject, name => partial doc
    - partial_by_sessiontype_crea...

Fix session import

See merge request !49

Readd an empty constructor for deserialization with Jackson. It was
accidentally removed a part of a code clean up in commit
685695a6.