Add a "document" style to PHUIRemarkupPreviewPanel and use it in Legalpad and Phriction
Summary: Ref T3671. Depends on D6674. Continues work in D6673, D6674 and extends it into Legalpad and Phriction. Then deletes a bunch of dead code.
Test Plan: Edited documents in Legalpad and Phriction, verified I got reasonable looking previews.
Reviewers: btrahan, Firehed
Reviewed By: btrahan
CC: aran, chad
Maniphest Tasks: T3671
Differential Revision: https://secure.phabricator.com/D6675