As an attendee getting free food, I should be unmarked as volunteering to pay for food

[kitsuta]

The situation is:

1. An attendee gets marked as wanting to pay for food ($25).
2. The attendee's badge then gets changed so they qualify for free food -- being given a badge type of Staff, for example.
3. The form displays no checkbox, just text informing the admin and the attendee that they get free food. HOWEVER, the attendee is still marked as paying for food in the system, and is still asked for money!

The fix is pretty simple -- we just need a presave_adjustment that checks to see if someone qualifies for free food, and if so, unmarks them as wanting to pay for food.