R1:7b0b820be123

Bridge GitHub users into Phabricator and attribute actions to them

Summary:
Ref T10538. Ref T10537. This creates PHIDs which represent GitHub users, and uses them as the actors for synchronized comments.

I've just made them Doorkeeper objects. There are three major kinds of objects they //could// possibly be:

- Nuance requestor objects.
- External account objects.
- Doorkeeper objects.

I don't think we actually need distinct nuance requestor objects. These don't really do anything right now, and…