Admins can add geographic regions ("locations")

* Create model and migration for locations
* Add route, controller, view, and partial for displaying a list of
all locations (Locations#index)
* Create an AdminController which checks that a user is an admin
* Add namespaced routes and Admin::LocationsController to handle #new
and #create actions (this controller inherits from AdminController, so
only admin users can access it)
* Add admin/locations#new view
* Add links to the application layout for "Home" and "Sign Out"

https://trello.com/c/JdYZB41D