Stripe Webhooks / Subscription Cancellation

* Disable repo if payments begin to fail due to expired card etc.
* Allows for future webhook integration

* Pulled the fixture (using `load_template`) from:
  * https://github.com/buttercloud/stripe_tester/blob/master/stripe_webhooks/2014-10-07/customer_subscription_deleted.yml

* Stripe webhook cheat sheet: https://www.petekeen.net/stripe-webhook-event-cheatsheet

https://trello.com/c/WulP3aqK
If it only deactivates active paid repos, I'd suggest renaming the method to something like `deactivate_paid_repos`.