Renaming Authentications to OmniauthCallbacks

In an implementation, when authentication was successful, I was
receiving the following message:

    translation missing: en.devise.authentications.user.success

The Devise translations were not showing up, as the controller name
was omniauth_callbacks and in the DeviseController#find_message,
the controller name's scope is automatically defined.

See related URL

https://github.com/plataformatec/devise/blob/v3.2.3/app/controllers/devise_controller.rb#L156