move ws handler to websocket package

add WebSocketMessage entity structure
add basic feedback logic
add tests for Feedback messages

Replaced the following events with StateChangeEvent:
* FeatureChangeEvent
* LockFeedbackEvent
* LockQuestionEvent
* LockVoteEvent
* LockVotesEvent
* PiRoundCancelEvent
* PiRoundDelayedStartEvent
* PiRoundEndEvent
* PiRoundResetEvent
* StatusRoomEvent
* UnlockQuestionEvent
* UnlockVoteEvent

Removed the following redundant events:
* DeleteAllLectureAnswersEvent
* DeleteAllPrepareationAnswersEvent
* DeleteAllQuestionsAnswersEvent
* DeleteAllQuestionsEvent
* LockQuestionsEvent
* UnlockQuestionsEvent
* UnlockVotesEvent

To trigger events, some code was migrated to use EntityServices instead
of directly accessing Repositories.

The entity is now stored in a separate property.

Additionally, roomId is used instead of Room object for legacy
RoomEvents.

@EventListener is used for handler methods instead of implementing
a visitor interface.

* Prefer simple IDs over entity types for method parameters
* Avoid use of obsolete v2 classes in service and repository layers
* Optimize imports

Consistently use singular for packages which contain mixed kinds of
classes. Moved a few classes to packages that better fit them.

Additionally, added AnswerController for API v3.

Rooms are now updated when Content is created or deleted.

Update handling is NYI.

Renamed UserAuthentication to ClientAuthentication and moved it to v2.
API v3 uses its own model.

When null was set as database key, the whole view would be requested
before this change.

A new cache 'room.id-by-shortid' has been introduced to speed up ID
lookups.

Some legacy identifiers remain very persistently.

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

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.

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`