Get details for email notifications from git repo instead of POST request
Created by: seanmadsen
When a book is updated, GitHub sends a webhook POST request to the app. The app then uses data from that request to (1) publish a specific book if necessary, and (2) send notification emails. We should consider the request as untrusted data. Therefor for step 2, we should not use data from the request. Instead, we should validate the
after refs from the request and use them to inspect the git repo locally and glean whatever info (e.g. email addresses, commit messages, etc.) is necessary for sending this notification email.