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