Factor out the Stripe code from the UserUpgrade class. Introduce a new
PaymentTransaction abstract class that represents a payment with some
payment processor, and a PaymentTransaction::Stripe class that
implements transactions with Stripe.
Note that we can't completely eliminate Stripe even though we no longer
accept payments with it because we still need to be able to look up old
payments in Stripe.
This upgrades from the legacy version of Stripe's checkout system to the
new version:
> The legacy version of Checkout presented customers with a modal dialog
> that collected card information, and returned a token or a source to
> your website. In contrast, the new version of Checkout is a smart
> payment page hosted by Stripe that creates payments or subscriptions. It
> supports Apple Pay, Dynamic 3D Secure, and many other features.
Basic overview of the new system:
* We send the user to a checkout page on Stripe.
* Stripe collects payment and sends us a webhook notification when the
order is complete.
* We receive the webhook notification and upgrade the user.
Docs:
* https://stripe.com/docs/payments/checkout
* https://stripe.com/docs/payments/checkout/migration#client-products
* https://stripe.com/docs/payments/handling-payment-events
* https://stripe.com/docs/payments/checkout/fulfill-orders
