Moved PI timer logic to its own service. The implementation for room
owner changes has been removed for now.

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.

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.