Add ids and events to notifications

[jkeen]

I wanted to be able to mark a notification record as read on the backend when a user closed a (frontile) notification on the frontend, and this PR opens up that possibility.

• Adds an id attribute to the notification object in frontile, and that id is output as a data-notification-id attribute on the Notification component.
• When a notification is added or removed on the frontend, an event is fired on the manager object, allowing consumers to tie into that.

An example of this in use:

...
  @service cable;
  @service notifications

  subscribeToUpdates(user) {
    this.subscribeToNotificationsChannel(user);

    this.notifications.manager.on('remove', this.notificationRead.bind(this));
  }

  notificationRead(notification) {
    if (this.subscriptions.notificationsChannel) {
      this.subscriptions.notificationsChannel.perform('mark_as_read', {
        ids: [notification.id],
      });
    }
  }
...