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
