Use relative URLs where possible

In general, we always use relative URLs (using `_path`), but sometimes
we were accidentally using absolute URLs (using `_url`). It's been
reported i might cause some isuses if accepting both HTTP and HTTPS
connections, although we've never seen the case.

In any case, this change makes the code more consistent and makes the
generated HTML cleaner.

app/controllers/users/omniauth_callbacks_controller.rb

@@ -36,7 +36,7 @@ class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
         set_flash_message(:notice, :success, kind: provider.to_s.capitalize) if is_navigational_format?
       else
         session["devise.#{provider}_data"] = auth
-        redirect_to new_user_registration_url
+        redirect_to new_user_registration_path
       end
     end