Set up the application layout for an admin app

This adds the name of the app and a sign out link to the navbar.
Change flash[:notice] to flash[:success]

This changes the key for flash success notices to be more explicit around
the nature of the notice, and matches the Bootstrap notice classes so 
that they appear in green.
Display the list of topics as a table
Tidy up the alert classes in a helper
Merge pull request #11 from alphagov/admin-template-tweaks

Style tweaks to the app