Namespace admin actions separately from user actns

* Create AdminController with before_action to ensure a user is an admin
* Remove #new action from LocationsController
* Create admin/LocationsController with #new action
* Add namescaped route for admin/locations#new
* Move locations#new view to admin directory
* Add is_admin? helper method to User model
* Add Sign Out link to application layout
Add create action for locations
Minor formatting tweaks