Evaluate deprecation of spree_user_return_to

[kennyadsl]

It's not clear if we really need to keep the custom spree_user_return_to session key.

We used it in https://github.com/solidusio/solidus_auth_devise/blob/b90a7f8e91d337637804a940adb9b8e6fe7343d4/lib/controllers/backend/spree/admin/user_sessions_controller.rb#L42 and https://github.com/solidusio/solidus_auth_devise/blob/b90a7f8e91d337637804a940adb9b8e6fe7343d4/lib/controllers/frontend/spree/user_sessions_controller.rb#L52, storing it at each store visit via the Spree::UserLastUrlStorer class.

I think it's to evaluate the deprecation of this custom session value (and setter class) since we can probably just use redirect_back(fallback_location:) provided by Rails since version 5.0.