R1:e5427a95211f

Extract GitHub actor IDs from GitHub events

Summary: Ref T10538. This probably gets push events where GitHub does not recognize the author wrong, but I don't have any of those yet.

Test Plan: Added and ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10538

Differential Revision: https://secure.phabricator.com/D15540