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.
The two methods we're using here are ``session.merge`` which takes e.g. an ``Attendee`` object which you instantiated manually and then adds it to the database.  This is different than ``session.add`` since saying ``session.add(foo)`` communicates that this is a new object which needs to be created, whereas saying ``session.merge(foo)`` communicates that this is an existing object already in the database which you'd like to attach to this session; the ``merge`` function then returns a new object which is the same thing but attached to the session.

Saying ``session.refresh(foo)`` says that we want to query the database for all of the latest values and just overwrite any values we have with whatever the database has.

In this way we're able to take our sessionized dicts which represent all of the paid preregs and just grab their latest values out of the database.