* Prefer ids over "foreign" domain objects.
* Consistently use final keyword for implementation parameters and
  variables.
* As a consequence, session id is used instead of the object as cache
  key for `skillquestions`, `lecturequestions`, `preparationquestions`
  and`flashcardquestions` caches.
* Rename occurances of `questionId` to `contentId`.

* 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`