Configure Devise to use custom Session views.

- Prohibit DirectoriesController from routing to /admin pages.
- Add /common/sessions partial.

Current spec status: 40 examples, 0 failures in 0.43801 seconds.
Add Admin::RegistrationsController to close sign-ups.

Current spec status: 44 examples, 0 failures in 0.36096 seconds.
Merge pull request #4 from sleepingkingstudios/feature-authentication

Close registrations, configure Devise views.