Lift handling of Herald "email" effect to Adapter

Summary:
Ref T7731. Every adapter subclass currently implements this effect in an essentially identical way.

Some day far from now the effects will be modular and this mess will vanish completely, but reduce its sprawl for now.

Test Plan: I'll test this thoroughly at the end of the change sequence since writing rules is a pain.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7731

Differential Revision: https://secure.phabricator.com/D12268