Apply Source Sans Font to PHUI Document Preview Skin
Summary: Fixes T5767
Test Plan: Tested Phriction, Legalpad. Edited a few documents.
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T5767
Differential Revision: https://secure.phabricator.com/D10779