Adding in an edit icon on the action header

Summary: Test flight on Phabricator editing! I mostly looked at other code, thought it was well written and wrote my own code in the other code's image.

Test Plan: Look at icons appearing!

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7911