Send Markdown in Diffusion through SourceSans
Summary: This moves Markdown rendering from normal fonts to PHUIDocumentView with Source Sans improving readability of this longer form text.
Test Plan:
Test libphutil and Phabricator readmes in my sandbox.
{F363483}
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D12330