Monospaced preferences now display in Remarkup areas

Summary: Fixes T6109, Ref T6654. Correctly clobbers the Remarkup defaults.

Test Plan:
Tested with Copperplate!

{F239347}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6109, T6654

Differential Revision: https://secure.phabricator.com/D10905