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