[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