Content Packs: Selecting all entities of a specific type in selection list, does not always select all entities.

## Description

On the content pack create page you can select, for example, all dashboard by clicking on the checkbox next to "Dashboard".

In the next step `Parameters` you will see an overview of all attached entities.

During my tests the `Parameters` step did not always list all entities. Sometimes it did not even show any entities. In theory you should not be able to get to this step without selecting entities.

I was only able to reproduce this when the dashboard list contains e.g. ~60 dashboards. With just a few dashboards this does not seem to be a problem.

## Environment

- Graylog Version: 6.3, but likely also older versions

## Checklist
[x] This issue fix need to be backported.
[] Does this issue have **security** implications?