docs: def set_short_url(self):
          """
          Generates the ``short_url`` attribute if the model does not
          already have one. Used by the ``set_short_url_for`` template
          tag and ``TweetableAdmin``.
  
          If no sharing service is defined (bitly is the one implemented,
          but others could be by overriding ``generate_short_url``), the
          ``SHORT_URL_UNSET`` marker gets stored in the DB. In this case,
          ``short_url`` is temporarily (eg not persisted) set to
          host + ``get_absolute_url`` - this is so that we don't
          permanently store ``get_absolute_url``, since it may change
          over time.
          """
          if self.short_url == SHORT_URL_UNSET:
              self.short_url = self.get_absolute_url_with_host()
          elif not self.short_url:
              self.short_url = self.generate_short_url()
              self.save()