R1:5b4825cf1eea

Use new DocumentView for Legalpad previews

Summary: Moves to showing Legalpad previews using PHUIDocumentViewPro

Test Plan: Create a new document, edit an existing document

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14550