Preserve newlines in remarkup previews

Summary: Fixes T10177

Test Plan: Saw that the preview of the rendered task description showed newlines proper.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T10177

Differential Revision: https://secure.phabricator.com/D15068