Add an endpoint suitable for a GitHub organization webhook

This is good because it means you can get a user checked /immediately/
on their being added to the Organisation, by setting the org webhook to
hit something like:

https://gu-who.herokuapp.com/audit/guardian?access_token={{ YOUR_GITHUB_API_KEY }}

Some possible future enhancements:

* Respect the payload delivered by the webhook - ie read member.login to
  work out which single user to check
* Support a JSON response on the output, rather than the ugly html
* Get gu-who to set the hook up for you?!

https://github.com/blog/1933-introducing-organization-webhooks