fixed season pass code for previous season supporters

I totally forgot about this, which was annoying; this used to foreign key into the ``Attendee`` table, but now it's a generic if field which can refer to one of multiple other tables.  But I left the foreign key constraint in place.

Annoyingly, I had to manually drop the table in production to drop this foreign key constraint.  People will need to do the same on their dev boxes - sorry about that.