Art Show sales form is passing through floating-point errors

[kitsuta]

We discovered at-con that, for some attendees, the Art Show sales form (/art_show_admin/pieces_bought) is passing floats, not integers, to its page handler. Said floats would look like, say, "59.00000025", indicating floating-point math errors. We fixed the issue using aggressive casting on the backend (see #525 and #527), but we need to figure out where we're doing math with floats and switch that to using integers.

[kitsuta]

We should consider investigating this as a must-have as it might be a sign of something serious. Investigation estimate: 1-2 hours