Child entities are now deleted by an event handler of their service
class.

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.

* 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.

This enables the manipulation of Rooms (shortId, ownerId) before
creation/updating for API v3.

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

Some legacy identifiers remain very persistently.

Additionally, remove obsolete VisitedRoom repository and renamed
VisitedRoom references to RoomHistory.

Implemented migration of settings.

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

* 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`

InterposedQuestion has been renamed to Comment. Method names have not
been touched yet to ease reviewing the changes.

Exceptions are now always passed to the logger. The logger decides based
on configuration whether and how they are logged. Logging messages were
adjusted for consistency.

Additionally, remove unused loggers.

* Remove unused code
* Optimize imports (group 10+ and static imports)
* Remove redundant variable assignments
* Replace explicit type parameters with <>
* Remove redundant catch blocks
* Use varargs instead of explicit array instantiation
* Weaken visibility of methods and fields
* Simplify conditions
* Do not use StringBuilder for simple concatenations
* Fix code style
* Remove invalid Javadoc

This simplifies analysis of monitoring data.

fix bug where abstention questions are not exported

implement: via two booleans you can now decide whether AnswerStatistics and FeedbackQuestions shall be exported, too