The Netty version set by Spring IO platform's BOM is not compatible with
netty-socketio.

Add support for CouchDB Mango queries

See merge request !72

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.

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.