On login forms, autofocus the "username" field

Summary: Depends on D20120. Fixes T8907. I thought this needed some Javascript nonsense but Safari, Firefox and Chrome all support an autofocus attribute.

Test Plan: Loaded login page with password auth enabled in Safari, Firefox, and Chrome; saw username field automatically gain focus.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T8907

Differential Revision: https://secure.phabricator.com/D20122