Summary:
- Fixes#1239
- Adds action in composer view to indicate when draft partcipants have
changed. This seemed like the simplest way to listen for this change without
adding another extension point
- Updates signature plugin to listen to this action and update signature
accordingly
- Adds test
Test Plan: - Unit tests
Reviewers: evan, bengotow
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D2614
