An admin can mark and unmark users as RMs or RAs

* Implement namespace routing to have admin functions
* Admin controller to ensure a user is an admin
* Admin directory (controller/views) to house the admin functions
* Add custom validation in User model
I'd be interested to see if @hrs thinks this functionality should be in a separate resourceful controller and action (e.g. `ResidentManagers#create` / `ResidentAssistants#create` or `ResidentRoles#create`) since it doesn't really update the whole user, but it just used to set the role. 

In any case it looks good and we can perhaps refactor it afterwards. Onward!