R1:eec593e3139b

Add ButtonBar icons

Summary: Adds an icon set for icons on button bars.

Test Plan: photoshop

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4394

Differential Revision: https://secure.phabricator.com/D8170