Always pre-wrap code blocks

Summary: Fixes T11416. Unclear what the side-effects of this would be, so bark if you find something. Previously, we'd have to overflow and scroll, which is kind of a pain since you're hiding content on long code blocks. This just wraps long lines, and preserves line breaks globally.

Test Plan: Test feed, profile, comments, inline comments, triple backticks.

Reviewers: avivey, epriestley

Reviewed By: avivey, epriestley

Subscribers: Korvin

Maniphest Tasks: T11416

Differential Revision: https://secure.phabricator.com/D16361