Tighten Remarkup and Fontkit CSS rules in Phabricator

Summary: Fixes T5497. Scope these down a little bit so they don't bleed into {W...} embeds and such.

Test Plan:

• Viewed a Legalpad document with headers, monospaced stuff, and lists. Looked the same before/after.
• Viewed a comment with headers, monospace, and lists. Looked the same before/after.
• Viewed a {W..} embed, now looks sane.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5497

Differential Revision: https://secure.phabricator.com/D9757