Make "/" focus the search input again

Summary:
See D1902, T989, T11263, D15984, T4103 , D15976, https://secure.phabricator.com/w/changelog/2016.22/, T2527, T11231, T8286, T11264 for discussion!

When we get another copy of T989, I will rename it to "Build a complicated keybinding settings page like a cool video game" and leave it open forever.

Test Plan: Pressed "/" in Firefox, had my pristine browsing experience inexplicably hijacked by this horrible application.

Reviewers: avivey, chad

Reviewed By: avivey

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15984