Add a model to store the status of user upgrades.
* Store the upgrade purchaser and the upgrade receiver (these are
different for a gifted upgrade, the same for a self upgrade).
* Store the upgrade type: gold, platinum, or gold-to-platinum upgrades.
* Store the upgrade status:
** pending: User is still on the Stripe checkout page, no payment
received yet.
** processing: User has completed checkout, but the checkout status in
Stripe is still 'unpaid'.
** complete: We've received notification from Stripe that the payment
has gone through and the user has been upgraded.
* Store the Stripe checkout ID, to cross-reference the upgrade record on
Danbooru with the checkout record on Stripe.
This is the upgrade flow:
* When the user clicks the upgrade button on the upgrade page, we call
POST /user_upgrades and create a pending UserUpgrade.
* We redirect the user to the checkout page on Stripe.
* When the user completes checkout on Stripe, Stripe sends us a webhook
notification at POST /webhooks/receive.
* When we receive the webhook, we check the payment status, and if it's
paid we mark the UserUpgrade as complete and upgrade the user.
* After Stripe sees that we have successfully processed the webhook,
they redirect the user to the /user_upgrades/:id page, where we show
the user their upgrade receipt.
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
