Fix mobile menu for files in Differential Changesets

Summary: Fixes T11305, Ref T7754. Makes this menu dropdown act like actions and collapse to a fa-bars menu.

Test Plan:
View on mobile, desktop, browser. Click an action, spawn new page.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T7754, T11305

Differential Revision: https://secure.phabricator.com/D16265