refresh session attributes before passing preregs to template

This is something that affected Anthrocon and will be a nice to have for us for futureproofing.
Here's what's going on here.

Preregistrations are not stored in the database until they've been paid for.  Until that time, we serialize them to a dict and store them in the CherryPy session.  The ``Charge.from_sessionized`` method takes those dicts and turns them back into model objects which we can pass to templates.

That all works.  However, we were doing the same thing for our already-paid-for registrations.  Which means that we only have the data that was used to save the registration to the database and NOT the fields which were generated afterwards (e.g. the ``registered`` field or the ``badge_num`` of a Supporter, etc).

This is fine for the information we're currently displaying, but Anthrocon wanted to display things like ``badge_num`` in the template, and maybe we will want to do the same thing someday.  In that case it would be very unintuitive to have data which is different from what's in the database.