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

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

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

Rooms are now updated when Content is created or deleted.

Update handling is NYI.

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`

(Lecturer/Skill)Question has been renamed to Content. Method names have
not been touched yet to ease reviewing the changes.

* 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

Events for all learning progress related actions
are generated, like answering a new question.
The event classes are used as a tag to evict
the cache based on the session.

Updated copyright year and adjusted header format to be consistent over
all files and with other ARSnova projects.