Creating subscription via api doesn't use pay result immediately

Up until now, sca has never trusted pay's status notifications (as there isn't a way to trust them), but with the unattended payments enabled in bug 1087752, the status of the payment is guaranteed to be one of ERROR, REFUSED, PENDING or AUTHORISED (where PENDING also means something went wrong).

This means that in the case of unattended purchases via sca's api, we can respond directly, either raising an error, or transitioning the sca subscription to PaymentAuthorised directly so the payment-notification-processor can process it much more quickly (ie. no need to wait for notifications from pay and then re-getting payment from pay).

Note: this came out of a review discussion with Ricardo for the work on bug 1087752: https://pastebin.canonical.com/80605/