Sentry

Done in 3 separate commits for clarity:
1. 19cdf7b - Drop in replacement of Sentry where Auth was previously used. After this commit, "php artisan seat:reset" must be run to recreate the admin account and password. Also includes a first pass at permissions within the key panel, where non-superusers can only see their own keys. This obviously does not extend to the other panels yet.
2. 6202603 - Renaming of UserController (which was responsible for logins) to SessionController, to make way for the next commit. The username field on the login view has also been relabeled as an email address field to disambiguate the code.
3. 89b3a99 - Addition of a UserController which is now responsible for user management. This controller is behind a filter that ensures the logged in user is a superuser.

Note: All users added via the user panel are equal administrators, meaning any one can remove the others. There is a check such that the last admin standing is unable to remove himself.