Pro: Cancellation Reasons

[garethrees]

Stripe now has more support for collecting cancellation reasons after a subscriber cancels their subscription. These get recorded in Stripe via Billing > Subscriptions > subscription details page.

After the user cancels, we should display a form that requests their reasons. The options given in the Stripe documentation are fine.

If we can use something pre-made from Stripe, all the better, but writing our own form and submitting via the API would be fine.