Modularize Application Email Herald field

Summary:
Ref T8726. There's no interface we can check for this, so the adapter needs to opt in.

Also fix a spelling mistake.

Test Plan: Created rules with "Application Email" fields.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13499