Allow users to set a line-height in their monospaced font preference

Summary: Ref T10959. This does not fix the problem because the .differential-diff td rule is still stronger, but it does let you choose a more compact or breezy style for remarkup blocks and pastes.

Test Plan:

• Set font to 24px / 48px impact.
• Viewed a paste, saw lovely readable text.
• Viewed an inline code block which was very easy on the eyes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10959

Differential Revision: https://secure.phabricator.com/D15904