Refactor architecture layers

See merge request !71

* skillquestions -> contentlists
* lecturequestions -> lecturecontentlists
* preparationquestions -> preparationcontentlists
* flashcardquestions -> flashcardcontentlists
* questions -> contents
* answers -> answerlists
* learningprogress -> score

With the use of JSON views they are no longer necessary.

Unfortunately, Spring Security cannot easily be used with our WebSocket
implementation since connections share a Thread and therefore share a
SecurityContext. An AuthenticationToken for WebSocket access to the
service layer is created.

* Remove redundant `isAuthenticated()` checks
* Move in-method checks to annotations where possible

It is no longer necessary since the creator property is not serialized
for the API.

Moved logic operating on multiple repositories to service layer.

Implemented a patch method which merges an existing entity with updated
properties from a HashMap. A new test dependency spring-security-test
has been added.

* Use consistent permission for all target domain objects:
    * read, create, owner, update, delete
* Use switch blocks to improve readability
* Fetch target domain objects early to reduce code duplication

Repositories now implement Spring Data interfaces. Basic CouchDB
operations are now implemented once by `CouchDbCrudRepository` instead
of having redundant methods to handle each design document.

* Prefer ids over "foreign" domain objects.
* Consistently use final keyword for implementation parameters and
  variables.
* As a consequence, session id is used instead of the object as cache
  key for `skillquestions`, `lecturequestions`, `preparationquestions`
  and`flashcardquestions` caches.
* Rename occurances of `questionId` to `contentId`.

Added new service for comments.

* Moved classes from `de.thm.arsnova` root package.
* New util package
* Do not start interface names with `I`. Appended `Impl` to the
  implementing class instead.
* Renamed learning progress classes to score and move them from
  `.domain` to a new `.services.score` package.
* Do not capitalize acronyms in class names:
    * CASLogoutSuccessHandler -> CasLogoutSuccessHandler
    * SocketIO* -> Socketio*
* Renamed `.socket` package to `.websocket`

Our version of CouchDB4J has been updated to fix and improve type
handling for database keys. Strings are no longer converted implicitly
to integers or booleans based on their content. This lead to bugs if key
and property values were compared.

Fix registration of Bean for socket event handling

See merge request !70

Fixes a regression introduced by commit
2ecbfec8.

Fix design doc creation

See merge request !69

Authentication is needed if an CouchDB admin user exists for design doc
creation at startup.

Closes GH-6.

Migration to Ektorp and refactoring of the database layer

A summary of the changes:
* CouchDB4J has been replaced by Ektorp.
* CouchDBDao has been split up into multiple repositories: One for each
  entity.
* Jackson is used instead of JSON-lib for POJO <-> JSON
  (de)serialization.
    * Jackson's `@JsonView`'s are used to mark which properties are
      (de)serialized for the API and/or CouchDB
    * A lot of persistence code could be simplified since serialization
      is handled automatically by the `ObjectMapper`.
* Entities and related Classes have been renamed:
    * `Content` <- `Question` / `SkillQuestion` / `LecturerQuestion`
    * `Comment` <- `FeedbackQuestion` / `AudienceQuestion`
* CouchDB design docs can now be automatically created by the backend if
  necessary. They are included as JavaScript code which is parsed and
  transformed to JSON at startup using Java's `ScriptEngine`.
* `MediaType`s for API versioning have been introduced:
    * `vnd.de.thm.arsnova.v2+json`: the format as used in ARSnova 2.x
    * `vnd.de.thm.arsnova.v3+json`:
        * ISO 8601 dates instead of timestamps
        * `id` instead of `_id`, `revision` instead of `_rev`
* `api.indent-response-body` can be set in configuration for pretty
  responses for API debugging.
* Empty (e.g. `null`) entity properties are ignored for serialization

See merge request !68