Refactor on-login Legalpad document signature requirement

Summary: Depends on D18786. Ref T13024. I'm going to change the order this occurs in, but move it to a separate method and clean it up a little first.

Test Plan: Added a new document as required, reloaded, signed it, got logged into a session.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13024

Differential Revision: https://secure.phabricator.com/D18788