Add multi-factor auth and TOTP support

Summary:
Ref T4398. This is still pretty rough and isn't exposed in the UI yet, but basically works. Some missing features / areas for improvement:

  - Rate limiting attempts (see TODO).
  - Marking tokens used after they're used once (see TODO), maybe. I can't think of ways an attacker could capture a token without also capturing a session, offhand.
  - Actually turning this on (see TODO).
  - This workflow is pretty wordy. It would be nice to calm it down a bit.
  - But also add more help/context to help users figure out what's going on here, I think it's not very obvious if you don't already know what "TOTP" is.
  - Add admin tool to strip auth factors off an account ("Help, I lost my phone and can't log in!").
  - Add admin tool to show users who don't have multi-factor auth? (so you can pester them)
  - Generate QR codes to make the transfer process easier (they're fairly complicated).
  - Make the "entering hi-sec" workflow actually check for auth factors and use them correctly.
  - Turn this on so users can use it.
  - Adding SMS as an option would be nice eventually.
  - Adding "password" as an option, maybe? TOTP feels fairly good to me.

I'll post a couple of screens...

Test Plan:
  - Added TOTP token with Google Authenticator.
  - Added TOTP token with Authy.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4398

Differential Revision: https://secure.phabricator.com/D8875