Make sure group_id is set instead of group

Fixes #469.
Here's an excerpt from the Google Chat conversation that Vicki and I had, which will help explain what's happening here and why this fix works:

The way the Django ORM handles foreign keys is that there will be an id field and a relationship field.  So like the Attendee objects has a group_id field, which is an integer because it's a foreign key into the Group table, and a "group" field.  If you say "attendee.group" then it'll load the group object by querying the database getting the value of the group_id and querying for it in the Group table.  So if you want to instantiate an Attendee object and set what group it belongs you, you can set either attribute; you can either say like "attendee.group_id = <some integer>" or "attendee.group = <some group object>".

So what seems to be happening is that in the credit card processing code, we serialize an Attendee object (which has a group) and stuff it in the session, then we grab it back out of the session and re-instantiate it.
So it looks like the bug is that we're assigning the group id to the "attendee.group" field instead of the "attendee.group_id" field.

So a somewhat kludgey way to stop the error from happening is to make sure we're setting "group_id" instead of "group".

So what's probably happening is, we're using a Django built-in method to serialize our model objects, but NOT using a Django built-in method to un-serialize them, which is what's causing the problem.
So there are two ways we could have tried to fix this:

1) Rethink how we're doing either serialization or parsing.  Apparently our approach is kind of fraught.  (I actually did rethink it for M13, which is why we don't have this problem there, but what I'm doing won't work for us since I'm using SQLAlchemy.)

2) Just fix this one issue and not worry about it, since this is probably the only problem with the way we're doing this, and this code ha already been replaced in UberPrime anyway.

I recommended the second approach, and Vicki agreed, so so basically we're saying, "remove the 'group' field from this dictionary if it exists, otherwise just return None' on the first line.  Then the second line says 'if there's not already a field in the dictionary called 'group_id', set it with the value we got from the 'group' field.