* Add a frequently asked questions section.
* Add nicer looking upgrade buttons.
* Format the page nicer.
* Prevent users from attempting invalid upgrades on users that are
already Platinum or above.
* Refactor to move upgrade logic from UserPromotion to UserUpgrade.
* Send the recipient and the purchaser of a gifted upgrade separate
dmail notifications.
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.
