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
You may want to refactor this into a helper method `toggle_resident_manager(user)`. The helper method will output the appropriate link based on `user.resident_manager?`. This will remove some of this logic from the view.