Bug 1067647 - Attach pull requests to bugs

* Attaches a pull request to bugzilla after a notification from the github webhook.
* Listens to the pulse amqp simple bugzilla exchange.
* On bug updates, if we are "subscribed" to a bug in memory (from a pull request detection), listens for an R+ flag on a github attachment.
* Merges a pull request if it receives R+ and has checkin-needed, and comments in the bug with the SHA.
* Removes checkin-needed from the bug and comments with the commit.

Example of an auto-landed bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1068317
why underscore?