Fix an issue with Duo not live-updating properly on login gates

Summary:
See https://discourse.phabricator-community.org/t/duo-broken-in-2019-week-12/2580/.

The "live update Duo status" endpoint currently requires full sessions, and doesn't work from the session upgrade gate on login.

Don't require a full session to check the status of an MFA challenge.

Test Plan: Went through Duo gate in a new session, got a live update.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20347