R1:cd6b28467829

Highlight strong tag in document headers

Summary: Fixes T12357. Adds some color to highlighted text in headers.

Test Plan: == Header with **strong** copy ==

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12357

Differential Revision: https://secure.phabricator.com/D17469