ligoauth.models.AuthGroup is an enhanced version of the builtin Django
Group model. We add useful attributes like a description, ldap_name,
and Tag.  These can be used to abstract things that are presently
handled manually, like inheriting membership from an LDAP group or
allowing access to view log messages/files with a specific Tag.

I ran some tests comparing this multi-table inheritance setup to
adding a OneToOneField and loading it with/without select_related.
After 1K trials, the DB query times looked comparable and didn't
require multiple queries for this arrangement, so the convenience
seems to be worth it.

The commit creates the model and populates an instance for existing
each existing Django Group.