Action Menu (mobile)

Summary: First pass here, still needs some UI work (maybe? tablets?). Getting it out to see if implementation is corrrect.

Test Plan: Shrink Maniphest task on Chrome, see new UI. Toggle menu. Test on iOS.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5631