Add a "documents I've signed" view to Legalpad

Summary:
Ref T3116. Allow documents to be queried for ones the viewer has signed, and make this the default view.

This also relaxes the versioning stuff a little bit, and stops invalidating signatures on older versions of documents. While I think we should do that eventually, it should be more explicit and have better coordination in the UI. For now, we'll track and show older signatures, but not invalidate them.

I imagine eventually differentiating between "minor edits" (typo / link fixes, for example) and major edits which actually require re-signature.

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D9769