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