Generate QR codes for TOTP tokens

Summary: Ref T4398. I found a reasonable-ish LGPLv3 library for doing this, which isn't too huge or unwieldy.

Test Plan:

• Scanned QR code with Authy.
• Scanned QR code with Google Authenticator.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4398

Differential Revision: https://secure.phabricator.com/D8923