Add github integration

[shaydewael]

Send message to chat with links to PRs when they open

[nickysemenza]

1. run express server in sadbot, point github webhook to that
2. when sadbot receives request from github, send to all the threads that are in like settings.admin_threads or something (so it can include sadboyz, and maybe all of the threads that include all of us + sadbot, if we want to add in more administrative notifications, etc.

thoughts on this methodology?

[nickysemenza]

now that @kedarv added an express server that's working for tumblr callbacks , we could do github hooks with even less effort