Notifications

Problem/Motivation

A big piece of missing functionality is around notifications, which are a regulatory requirement. We need to:

(1) Identify all circumstances in which notifications are required, and the different types of notifications.
(2) Create an event for each type of notification
(3) (probably) Create a NotificationDispatcher service to centralise logic around when/if to dispatch the notification
(4) Create an EmailNotifier service that sends out email notifications.
(5) Create an event subscriber that calls on the EmailNotifier.
(6) Probably use twig templates as the defult text for the notifications. What Commerce does here with emails is likely a good pattern to copy.

Tricky questions:
(1) How do we get the email address for the EmailNotifier. It's different for different declaration contexts.
(2) How do we avoid duplicate notifications when a declaration falls under multiple contexts
(3) How do we satisfy any audit requirements for proving we have sent the notification

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes