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.
- Replace deprecated methods on client
- Fix broken dropdowns due to select kit 2 upgrade
- Graceful error handling when Stripe keys are not configured but plugin enabled