fix(login) - Override devise user registration

De-activate :validatable module of devise:
* Validation of email will always fail, since we don't store
  domain name as part of email field.
* Validation during registration will fail since a confirm-password
  screen isn't provided at the getting started screen.

Also, the all registration requests will be re-routed to the
registrations_controller, so as to save other fields such as
department_id, phone, avatar, etc.