[Redesign] Clean up differential file headers
Summary: Ref T8099, normalizes font-weight on the Differential file headers.
Test Plan: Review a diff in sandbox for correct font weight.
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T8099
Differential Revision: https://secure.phabricator.com/D13438