[Redesign] Update PHUIRemarkupPreviewPanel

Summary: Ref T8099, Adds DocumentView support, and cleans up headers.

Test Plan: Review preview in Legalpad, Phriction, and Maniphest.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13442