Simplify "did this person pay" logic

[dehnert]

There are various places where we compute "did this person pay for this past dance". It's a bit of a mess, and also doing it correctly (even just for expected changes) requires looking at the Person history to determine the fee category at the time. We should really simplify this (see TODO on the Attendee model), and then the various places that do this computation should be simplified themselves. (Going forward, I'll try to tag them with "TODO: #28 ...".)