[Redesign] Give a default line height on codeblocks
Summary: Ref T8099, Gives a default line height to codeblocks, slight background color to pastes.
Test Plan: Tested diffs, pastes, codeblocks in remarkup
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T8099
Differential Revision: https://secure.phabricator.com/D13470