#29: person registering the group can now kick in extra

The whole "amount_extra" thing could use a lot more code comments and documentation, which I'll try to tackle one of these days.  Here's what's relevant for this ticket:
* groups have a cost and amount_paid
* group members have the special PAID_BY_GROUP paid status
* attendees with PAID_BY_GROUP badges can still kick in extra; in this case they have their own amount_extra and amount_paid values set appropriately
* Therefore, to properly make donations work for groups, we set the group cost and amount_paid to just the price of the badges, and register the donation under the badge of the attendee who kicked in the extra money, since we'd prefer that donation to be tied to that specific badge in case it gets transferred or something.