Legalpad - add "no one" signature type

Summary: Fixes T7294. This lets legalpad store other documents that don't need signatures but conceptually belong in legalpad.

Test Plan: made a document with signature type "no one" and it saved. viewed the document and noted no signing UI was present.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7294

Differential Revision: https://secure.phabricator.com/D11788