update reason1 field per sofort request

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`.

Please review @odorcicd