Require streams selection when creating an event definition

## What?
The streams select is currently optional, when creating an event definition.

If a user does not have access to all streams and leaves this field empty, the POST which creates the event definition results in a 403.

## Why?
We should require the streams selection to ensure a user is able to create an event definition and to avoid the following scenario:
1. User creates an event definition with a filter which does not include any stream
2. New inputs and streams get configured
3. Filter for the event definition also includes new inputs / streams, even though it probably should not

## Your Environment

* Graylog Version: 4.0