Allow Legalpad document managers to add signature exemptions

Summary:
Ref T5532. Allow document managers to add exemptions, which act like signatures but are tracked a little differently.

The primary use case for us is users who sign a corporate CLA and need a user-level exemption if they don't want to sign an individual CLA.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5532

Differential Revision: https://secure.phabricator.com/D9795