Add some authorization I18n messages
Fix Admin before action checking Admin user status

If a user is not an admin, redirect them back to the home page.
Optionally, we can flash a message stating that they do not have the
proper authorization to access the interface, but that may be revealing
too much information.
Generalize and extract AdminController functions

In anticipation of more Admin controllers being added, create a base
Admin controller from which all the others will inherit and add any
general attributes, such as which layout to use and the 'admin'
requirement for users to the new controller.