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