Set up notifications for System Events

## What?

I want to be able to set up notifications from messages that will be ingested on the `All system events` stream.
I see that exist an Event definition created with name `System notification events` that appears on the Graylog UI as events, but you cannot modify it to receive alerts though methods like Slack or Email.

I checked this "missing feature" and I see another tickets related https://github.com/Graylog2/graylog2-server/issues/7610 and https://github.com/Graylog2/graylog2-server/issues/15949 but not sure if you still consider this feature or not.

## Why?

This makes easier for teams that manages Graylog to track errors or configuration problems.

## Your Environment

* Graylog Version: 5.1.4
* Elasticsearch Version: 7.10
* MongoDB Version: 5.0.10
* Operating System: Docker version
* Browser version: Chrome