follow-up: update reason1 field per sofort request

[simondeichsel]

SofortBanking (formerly directebanking) has asked us to stop submitting the description field (shop.name - checkout.id) as the reason_1 field, as it's getting truncated, and is no longer unique. Instead we'll begin sending the checkout.id as the reason_1 field, via :order.

This is a follow-up to:
#81