Modernize Countdown

Summary: Crumbs, phts, and mobile layouts for Countdown.

Test Plan: Tested new, edit and delete timer. Verified timer works. Unable to get 'remove chrome' to work, will investigate.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5101