Allow Legalpad documents to designate corporate signers

Summary:
Ref T5532. This adds:

• Documents can designate that they should be signed by "Corporations" or "Individuals".
• Corporate documents get different fields and a different exemption process.
• Basically everything works the same but this is like a zillion lines of form code.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5532

Differential Revision: https://secure.phabricator.com/D9812