Begin bridging GitHub objects through Doorkeeper
Ref T10538. This sets up a Doorkeeper bridge for GitHub issues, and pulls issues from GitHub to create ExternalObject references.
Broadly, does nothing useful.
Test Plan: Put a var_dump() in there somewhere and saw it probably do something when running bin/nuance update --item 44.
Reviewed By: chad
Maniphest Tasks: T10538
Differential Revision: https://secure.phabricator.com/D15447