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.

{F171052}

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5497

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