- Moved ensure_current_user_is_superadmin! from  `Superadmin::BaseController` to `ApplicationController` since active_admin inherits from ApplicationController and doesn't find ensure_current_user_is_superadmin!
  resulting in an error.
- Added active admin dashboard controller test to verify:
1. Success response for admin page
2. Redirection for non superadmin user.
Merge pull request #8 from bigbinary/fix-active-admin-helper

Fixed activeadmin access