Redesign Legalpad

Summary: Rolls out PHUIDocumentViewPro to Legalpad. Minor tweaks to provide space around Preamble and Signature blocks. Otherwise, straight forward.

Test Plan:
Build a new document with and without Preamble, sign document.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14377