main(): Make endpoint to handle GitHub webhook

Convert our dummy test route to a real one that will handle webhooks
coming from GitHub. It will parse the POST body data and create a
`CommitRef` from it. That `CommitRef` then gets passed to
`find_and_track_build_and_update_status()` to update the pull request
status based on Jenkins' build results.

A 202 response seemed apt here. Quoting
https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html:

> 10.2.3 202 Accepted
>
> The request has been accepted for processing, but the processing has
> not been completed. The request might or might not eventually be acted
> upon, as it might be disallowed when processing actually takes place.
> There is no facility for re-sending a status code from an asynchronous
> operation such as this.

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