Fake an outline ring on buttons

Summary: Fixes T11990. Chrome/Ubuntu doesn't respect outline: auto, so this is the only workaround I could fine. Somewhat nicer than stock Chrome.

Test Plan: tab tab tab tab, ubuntu, chrome, windows, safari.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11990

Differential Revision: https://secure.phabricator.com/D17023