docs: def get_message_type_for_app(app_name, default_message_type_alias):
      """Returns a registered message type object for a given application.
  
      Supposed to be used by reusable applications authors,
      to get message type objects which may be overridden by project authors
      using `override_message_type_for_app`.
  
      :param str|unicode app_name:
      :param str|unicode default_message_type_alias:
  
      :return: a message type object overridden is so, or the default
      :rtype: MessageBase
  
      """
      message_type = default_message_type_alias
      try:
          message_type = _MESSAGES_FOR_APPS[app_name][message_type]
      except KeyError:
          pass
      return get_registered_message_type(message_type)