Differential Changeset header icons

Summary: Adds filetype icons, applying to differential file headers. The main issue is with all the lightening, I wanted something to still anchor 'new file' on the page and adding a sharp icons does that pretty well for me. Feedback is cool too.

Test Plan: Add some new icons, test in previous commits.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7320