Add a migration that South thinks is necessary

This deals with the issue described in #514 - we don't
really understand why this is required, but our best guess
is that previous migrations were generated with different
versions of South, some of which didn't care about
blank=True.  However, we don't have more time to deal with
this at the moment, so this at least allows me to create
clean new migrations with
./manage.py schemamigration core --auto