Legalpad - add a preview to document create / edit
Summary: 'cuz legal documents be long! Fixes T4382.
Test Plan: created / edited a document and got working preview
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T3482
Differential Revision: https://secure.phabricator.com/D6386