SQL: some corrections.
Server: JPA-Entities, Repositories

Impl. Antwort, Frage
Upd. Spiel, Spielstatus, Spieltyp, Freundesliste, Kategorie,
Kategorienfilter
Server: Impl. game overview
Server: FIXED CRITICAL security issue.

On syncing, the client does not get password, push-id, last-activity of
his opponent and himself, but only the user name.
Server: Remove unused.
Server: Mergeprobleme manuell behoben.
Merge branch 'master' of https://github.com/kevin-strobel/WI-Quiz

Conflicts:
	Backend/src/main/java/studiduell/controller/SettingsController.java
	Backend/src/main/java/studiduell/controller/SpielController.java
	Backend/src/main/java/studiduell/model/AntwortEntity.java
	Backend/src/main/java/studiduell/model/FrageEntity.java