Deprecate first order promotion rule for nth order

[seand7565]

The nth order promotion rule handles applying promotions to a users nth order. The only catch is, N can't be 1. The comment on the rule seems to indicate that the reason for this is that we already have a first order promotion rule. But given that there's no real distinction between first order and nth order where N=1, and a user should expect to be able to set N to 1, I would expect nth order to fully encompass first orders functionality.

In the interest of keeping the promotions system as light as possible, we could deprecate the first_order promotion rule in favor of the nth order rule. We'd need to move a small amount of logic from first_order to nth_order, but it seems like a small enough change, and since that logic only applies to N=1 - which currently is impossible because of the validations on the rule - we won't be breaking nth_order for anyone.

[waiting-for-dev]

I think it makes total sense.