Center button icons when on mobile displays

Summary: Fixes T8885. This view is restricted enough on mobile that we can widen and center the icon.

Test Plan: Test workboards, pinning apps, viewing action menus.

Reviewers: btrahan, joshuaspence, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8885

Differential Revision: https://secure.phabricator.com/D13647