Add "Send an email" action to Herald for Maniphest

Summary: Fixes T4403. Supports the "send an email" action in Maniphest.

Test Plan: Wrote a "email duck" rule, then commented on a task and saw "duck" get an email.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4403

Differential Revision: https://secure.phabricator.com/D8529