Autofocus the "App Code" input on the TOTP prompt during MFA gates after login

Summary: See downstream https://phabricator.wikimedia.org/T229757. The "autofocus" attribute mostly just works, so add it to this input.

Test Plan: As a user with TOTP enabled, established a new session. Saw browser automatically focus the "App Code" input on the TOTP prompt screen.

Differential Revision: https://secure.phabricator.com/D20703