#421: registering group members caused the group price to change

This is a subtle but important distinction.  Both of these are valid ways to assign an attendee to a group, but there's an important difference:
* when appending the attendee to ``group.attendees`` then the group will be aware of the attendee before the session is committed
* when just setting the foreign key, the attendee will only be registered as part of the group after the commit

So both of these statements are equivalent as far as the attendee is concerned, but the group needs to see the attendee BEFORE the commit, because we just dropped one of its badges and are adding this one to replace it.  So if we don't do things the correct way, it'll automatically re-adjust the badge price to something incorrect.