[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