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

Shows an error when a user is made both RM and RA

* Use toggle method in admin/usercontroller
* Display error in the index page, if any
* Pull logic from index to user_helper.rb