Fix bug 1005881: Move tweets cache to the DB.

This will fix wonkyness with the fact that our two datacenters have separate
memcached servers.
How about using [django_extensions.db.fields.ModificationDateTimeField](https://github.com/django-extensions/django-extensions/blob/master/django_extensions/db/fields/__init__.py#L223) instead of overriding the save method below?