Fix für Probleme im Lerngruppenmanagement

Fix für #63 (closed), #130 (closed), #145 (closed), #153 (closed), #155 (closed), #159 (closed) Außerdem sollten <>() in jeglicher erdenklichen Form bei lerngruppenrequests herausgefiltert werden.

requested review from @dnns01

merged manually

closed

mentioned in merge request !175 (merged)