Implement two factor authentication

If enabled, require a second factor to acquire a session cookie and
reject basic authentication attempts (as second factor cannot be
presented). Allow previous and next token for clock skew.

To enable this, you add a "2f" object to your user doc with a secret
"key" value.

This work is currently missing tests and documentation, and we'll
also want a UI (to render a QR code with an otpauth url for easy
setup).
The last sounds good - it shouldn't collide with any custom fields and we can always group auth-related fields later if they becomes too much.