Make old GitHub OAuth URIs work for now

Summary: Ref T1536. Like Google, GitHub is actually strict about callback URIs too. Keep them pointed at the old URIs until we can gradually migrate.

Test Plan: Logged in with GitHub.

Reviewers: garoevans, davidreuss, btrahan

Reviewed By: garoevans

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6265