Use: addGroup() and removeGroup() should return boolean

Use: addGroup() and removeGroup() should return boolean

Have User::addGroup() and User::removeGroup() methoids return a
boolean when their respective hooks return the respective boolean.

Fix SpecialUserrights to respect this return vale and update the
add/remove arrays accordingly.

This resolves an issue where a hook that prevents a group from
being added or removed still shows that group being changed in
the Userrights log.

Change-Id: I7621cc22b04ff41cf67bd434a1f89d31bdc2cffd