docs: def get_class_from_settings_from_apps(settings_key):
      """Try and get a class from a settings path by lookin in installed apps.
      """
      cls_path = getattr(settings, settings_key, None)
  
      if not cls_path:
          raise NotImplementedError()
  
      try:
          app_label = cls_path.split('.')[-2]
          model_name = cls_path.split('.')[-1]
      except ValueError:
          raise ImproperlyConfigured("{0} must be of the form "
                                     "'app_label.model_name'".format(
                                                                  settings_key))
  
      app = apps.get_app_config(app_label).models_module
  
      if not app:
          raise ImproperlyConfigured("{0} setting refers to an app that has not "
                                     "been installed".format(settings_key))
  
      return getattr(app, model_name)