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.
The code in the plugin needed a dramatic cleanup. This refactor collapses the Plan/Product/Subscription controllers on the backend into one new controller: `SubscribeController`.
This reduces N+1 calls to the back end during the subscription process and simplifies use of the code.
I've also removed a bunch of dead code and refactored some logic into methods for easier readability. No feature/functionality changes in this commit; only refactoring. However, refactoring will allow for implementation of better anonymous user handling, so this is largely a foundation to enable making that change.
Justin DiRose