Don't send notification emails if there are too many incoming communications

Formageddon email addresses get subscribed to members' mailing lists, which
leads to their spamming users that contact them via formageddon. We will still
accept and archive these communications, but won't notify users if there are
more than 3 communications from an office on a particular formageddon_thread.
Merge pull request #478 from sunlightlabs/notification_guard

Don't send notification emails if there are too many incoming communicat...