Feature request: Make e-mail contents editable by registering them as system messages

[boboldehampsink]

I.E.

        Event::on(SystemMessages::class, SystemMessages::EVENT_REGISTER_MESSAGES, function (RegisterEmailMessagesEvent $event) {
            $event->messages[] = [
                'key' => 'back_in_stock',
                'heading' => 'When a product is back in stock:',
                'subject' => 'Back in stock',
                'body' => '{{product}} is back in stock',
            ];
        });

Fully optional of course, but would be nice to have, especially for developers who can't do this themselves.