Stripe webhooks

[Mike Dewhirst]

Ticket #78 implemented Stripe subscriptions and left item 6 for this ticket as follows:

6. We need to establish a Stripe Webhook to receive event information such as:
   • customers upgrading their subscription to the next level -> re-provision access to that level
   • customers downgrading their subscription to a lower level -> re-provision access to that level provided token.used is fewer than the new token.package_size. Otherwise we need to get in touch.
   • customers cancelling their subscription -> de-provision access and get in touch to determine what to do with their data.

This will first require idempotency keys to prevent repetitive user activity from generating repetitive events. This implies a requirement for events which can be considered unique and thus worthy of having a unique idempotency key to attach to Stripe events.