Use Source Sans for Phriction

Summary: Adds Phriction to list of apps that use Source Sans as default font in addition to Legalpad and Diviner.

Test Plan: Tested various layouts imported from secure. Should be reasonably tested, but will follow up on secure.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10064