Schedules message for a delivery.
        Puts message (and dispatches if any) data into DB.

        :param list|None recipients: recipient (or a list) or None.
            If `None` Dispatches should be created before send using `prepare_dispatches()`.
        :param User|None sender: Django User model heir instance
        :param int|None priority: number describing message priority
        :return: a tuple with message model and a list of dispatch models.
        :rtype: tuple

def schedule(self, recipients=None, sender=None, priority=None):
        """Schedules message for a delivery.
        Puts message (and dispatches if any) data into DB.

        :param list|None recipients: recipient (or a list) or None.
            If `None` Dispatches should be created before send using `prepare_dispatches()`.
        :param User|None sender: Django User model heir instance
        :param int|None priority: number describing message priority
        :return: a tuple with message model and a list of dispatch models.
        :rtype: tuple
        """
        if priority is None:
            priority = self.priority

        self._message_model, self._dispatch_models = Message.create(
            self.get_alias(), self.get_context(), recipients=recipients, sender=sender, priority=priority
        )
        return self._message_model, self._dispatch_models