Require high-security session to sign legal documents

Summary: Ref T3116. If you have MFA on your account, require a code to sign a legal document.

Test Plan: Signed legal documents, got checkpointed.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D9772