This adds support for Stripe Promo Codes in the user checkout process.
Also adds a discounted field to User > Billing > Subscriptions to show the amount or percent discounted.
This does not currently add in support for creating promo codes in the Subscriptions interface (that will come at a later point in time). Instead a coupon can be created with a promo code right from the Stripe dashboard.
Previously, when a user canceled a subscription, the access would revoke
immediately on Discourse vs. at the end of the billing period. This
commit changes the behavior to remove membership at the end of the
billing period using Stripe's `cancel_at_period_end` attribute on the
Subscription object.
This commit now requires the setup of webhooks for subscription
processing to occur correctly.
Justin DiRose
Rimian Perkins