Meaning of empty streams input field when defining aggregation events is confusing

## Current Behavior
Given a user with access to only a limited set of streams
- creates an aggregation event definition with empty streams field
- receives error: insufficient permissions
The reason is that empty streams equates to all streams; and this user only has access to some.

## Expected Behavior
It would be more intuitive in my opinion, if specifying no streams equates to all streams _available to me_.
I may not be aware of any other streams that aren't shared with me. I just care about creating an event for all visible streams.

## Possible Solution
Implicitly assign the current set of available streams to the event definition (and show it explicitly the next time the user views or edits the event definition).
Add a check box for _all streams_, so we can continue to have an event definition that also applies to streams created in the future.

## Your Environment
* Graylog Version: 5.1.0