43617561: Admin login at /admin/login - redirected to /admin/qa_videos#index

* Admin seed data
* Added an /admin/login form
* Admins logging in at /admin/login are redirected to admin/qa_videos#index action. Logged-in admins visiting /admin are redirected to /admin/qa_videos
* Logged-out admins visiting /admin are redirected to /admin/login
* Admins who login at /login_or_register are redirected to /dashboard