Don't skip UserGetRights hook in addGroup/removeGroup

The addGroup and removeGroup functions in User were reloading the cached
mRights, but not calling the UserGetRights hook that is supposed to
allow for extensions to modify the rights list. Instead, let's just null
the cache so it will be reloaded next time something calls
$user->getRights().

Note we still call $this->getEffectiveGroups( true ), though, to trigger
that to recache.

Change-Id: I81784917303b639bc7c22c726e9cbdb0d191e674
Merge "Don't skip UserGetRights hook in addGroup/removeGroup"