fixed season pass code for previous season supporters

All automated emails are passed a single variable, which is the instance of the model being emailed.  So we name that variable the lower-cased version of the class name, so an instance of the ``Attendee`` class is passed with the name ``attendee``, etc.

Here I added a special case where ``PrevSeasonSupporter`` will also be passed as ``attendee`` because they're sent an email which is also sent for ``Attendee`` instances and it has all of the required fields.