A new service, repository and entity have been introduced to handle file
attachments to other entities. Attachments store meta data about the
file. The file upload is not yet implemented. It is planned to abstract
the storage handling to allow the support for different storage
solutions (local file system, database, cloud).

Additionally moved previously missed v2 entities to the v2 package.

Introduced content groups which replace question variants and support
sorting.

Ektorp, the library used as connector to CouchDB, currently does not
support features introduced with CouchDB 2.0. These changes implement
support for Mango API's `_find` which provides a more flexible
alternative to views for data retrieval.

See http://docs.couchdb.org/en/stable/api/database/find.html.

* 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`

Fixes a regression introduced by commit
2ecbfec8.

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

Closes GH-6.