Better element deletion

An element should delete its dependencies before deleting himself, helping the DB to solve its dependencies (this is a huge speedup).

Something like this seems to work:

def rm(e):
   children = Element.objects.get_descending(e.id)
   ids = children.values_list('id', flat=True)
   Classification.objects.filter(element_id__in=ids).delete()
   MetaData.objects.filter(element_id__in=ids).delete()
   ElementPath.objects.filter(element_id__in=ids).delete()
   children.delete()
   ElementPath.objects.filter(element_id=e.id).delete()
   e.delete()