Bring Duo MFA upstream
Summary: Depends on D20038. Ref T13231. Although I planned to keep this out of the upstream (see T13229) it ended up having enough pieces that I imagine it may need more fixes/updates than we can reasonably manage by copy/pasting stuff around. Until T5055, we don't really have good tools for managing this. Make my life easier by just upstreaming this.
Test Plan: See T13231 for a bunch of workflow discussion.
Reviewed By: amckinley
Maniphest Tasks: T13231
Differential Revision: https://secure.phabricator.com/D20039