Switch Order table to provide more annotated information

[melinath]

Right now, we have a status field on Order which has a few options: IN_PROGRESS, PENDING, COMPLETED, and REFUNDED.

This is okay, but we can / should do better. For example, we now fully support multiple transactions on an Order. So an order can be "completed" and still have a cart. Or it can be "refunded", in that it has a refunded transaction, but "completed" in that it has another transaction that is still valid.

I think the solution is that we should instead annotate the queryset with the payment / refund information and use that to determine what state the order is in. Ideally in a way that lets us still filter by "state", but remove the status field.

[melinath]

(this is waiting on the merge of #409 changes.)