All events stream - not disabled to create extractors

## Current Behavior
When you are looking at the search for the `All events` stream you habe the ability to select "create extractor" when you select this it looks like all should work normal but ends in a strange situation for the user:


## Expected Behavior
For events from the `All Events` or `System Events` Stream it should not be possible to create extractors. That should be disabled or give a meaningful error message.

## Steps to Reproduce (for bugs)
1. have events that are written to `all events` aka alerts
2. select any value and select `create extractor`

## Context
This community thread triggered my attention to this: https://community.graylog.org/t/can-i-create-extractor-on-all-event-stream/15619?u=jan

## Your Environment

* Graylog Version: 3.3.0