Begin bridging GitHub objects through Doorkeeper

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10538

Differential Revision: https://secure.phabricator.com/D15447