I feel like having to set the var, clear the session, then pass the stored value back again is muddying the `after_sign_in_path_for` method a little bit.

What if we did this?

    def get_and_reset_previous_location
      session.delete 'user_return_to'
    end

    def after_sign_in_path_for(resource)
      save_detected_locale(resource)
      return_path = get_and_reset_previous_location
      if invalid_return_urls.include? return_path
        dashboard_path
      else
        return_path
      end     
    end