Octocat for 'Land to GitHub'

Summary: Adds an action icon for GitHub.

Test Plan: I couldnt see how to update that icon in Differential. But. Photoshop.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7635