Make base font a Celerity CSS variable

Summary: For consistency, we switch back to base font in a few places when using alternate fonts like source-sans or monospace, this makes sure the base font is consistently reset.

Test Plan: Review a Document, a Diff, and a Legalpad form

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11891