Add OAuth2 authentication (#147)

- Added Site settings > Authentication section
- Create/edit/delete your custom oauth2 configurations
- Login or signup with oauth2

app/controllers/registrations_controller.rb

@@ -1,6 +1,7 @@
 class RegistrationsController < Devise::RegistrationsController
   # Needed to have Current.tenant available in Devise's controllers
   prepend_before_action :load_tenant_data
+  before_action :load_oauths, only: [:new]
 
   # Override destroy to soft delete
   def destroy