When an item is deleted, first delete any Activity object that has been created
      on its behalf.

def remove_orphans(self, instance, **kwargs):
      """
      When an item is deleted, first delete any Activity object that has been created
      on its behalf.
      """
      from activity_monitor.models import Activity
      try:
        instance_content_type = ContentType.objects.get_for_model(instance)
        timeline_item = Activity.objects.get(content_type=instance_content_type, object_id=instance.pk)
        timeline_item.delete()
      except Activity.DoesNotExist:
        return