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.
Add the following bank redirect payment methods:
* https://stripe.com/docs/payments/bancontact
* https://stripe.com/docs/payments/eps
* https://stripe.com/docs/payments/giropay
* https://stripe.com/docs/payments/ideal
* https://stripe.com/docs/payments/p24
These methods are used in Austria, Belgium, Germany, the Netherlands,
and Poland.
These methods require payments to be denominated in EUR, which means we
have to set prices in both USD and EUR, and we have to automatically
detect which currency to use based on the user's country. We also have
to automatically detect which payment methods to offer based on the
user's country. We do this by using Cloudflare's CF-IPCountry header to
geolocate the user's country.
This also switches to using prices and products defined in Stripe
instead of generated on-the-fly when creating the checkout.
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
