Modularize "Send an Email" Herald actions
Summary: Ref T8726. No surprises.
Test Plan:
Created rules using both action variants, applied upgrade, saw rules still work correctly.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: joshuaspence, epriestley
Maniphest Tasks: T8726
Differential Revision: https://secure.phabricator.com/D13701