Prevent group creation by silenced users.

Prevent group creation by silenced users.

* adds Right::CREATEGROUP
* logic in Profile::hasRight() checks for silencing
* NewgroupAction checks for the permission before letting you see or process the form in the UI
* User_group::register() logic does a low-level check on the specified initial group admin, and rejects creation if that user doesn't have the right; guaranteeing that API methods etc will also have this restriction applied sensibly.