Feature: Added action button to group/user list to add/remove group membership.

To implement this functionality following changes were necessary:
* Add new association table to keep track of wanted memberships.
* Changed prototype for item action authorization and change method to enable multiple parameters.
* Merge GuiUserModel with UserModel to make objects comparable.
* Implemented new base class for easy implementation of relation views.
* Implemented new endpoints for group membership management.
* Added new action buttons to users list in group detail view.
* Added new action buttons to group list in user detail view.

(Redmine issue: #5762)