Make event subscribers private by default

On my local install there are 625 services registered in the container. If I make most of the event subscribers private this drops to 551.

Needs 📌 Replace ContainerAwareEventDispatcher with Symfony EventDispatcher Fixed first because the Drupal event dispatcher does not support private services.

I think we could do something smart here and only make the service private if there are no public methods that aren't referred to by getSubscribedEvents().