Mark a sale as refunded

Marks a sale as refunded and reverses the commission attached to it.

If the sale was already refunded the request still succeeds, with alreadyRefunded set to true. A sale cannot be refunded once it has passed your program's refund window, has been added to a payout batch, has already been paid out, or has been marked as manually paid — each of those returns a 400 with a reason you can branch on.