Improve commit action item layout on mobile

Summary:
Ref T13552. Build the "commit list" elements so that the menu action items collapse under the element on mobile.

Also change the mobile breakpoint to 512px because my Safari window can't go any narrower than 508px. Future changes to responsive design will be more content-aware anyway.

Test Plan: Looked at commits in various interfaces, at desktop and mobile widths.

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21430