new model caching: fixing `delete_cache` methods.

Trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
applying new model caching mechanism to UserGroupMemberships.

Trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
including gitter.im channel in maintenance info box.