Add an optional preamble to Legalpad documents

Summary:
Fixes T5532. Allow documents to have a preamble in the header which can be used to explain who should sign a document and why.

Particularly, I plan to use this to navigate the corporate vs individual stuff more sensibly.

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5532

Differential Revision: https://secure.phabricator.com/D9819