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
I didn't notice this in my first pass, but now that I'm looking at it again I believe that in addition to calling `deactivate` on `repo` you'll want to delete the GitHub webhook and set the repo's `hook_id` to `nil`.

I also noticed that `RepoActivator` already has a `deactivate` method defined on it which deletes the hook and deactivates the repo. For code reuse purposes, consider renaming this class and using `RepoActivator#deactivate` within it.

Pseudocode:

```
BatchRepoDeactivation.run(stripe_customer_id)

class BatchRepoDeactivation
  def self.run(stripe_customer_id)
    repos = repos

    repos.each do |repo|
      RepoActivator.new(github_token, repo).deactivate
  end
end
```