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
