Extract property-whitelist into constant for easier comprehension of relation

[Joshua Behrens (JoshuaBehrens)]

1. Why is this change necessary?

In the past it was not easy to understand what the property-whitelist does and where it comes from. Seeing the relation of the cms loader running first, changing the incoming request and the later feature subscriber using that data is not very easy to understand. This makes it easier.

2. What does this change do, exactly?

Introduces a constant and used it everywhere applicable.

3. Describe each step to reproduce the issue or behaviour.

1. Try to understand how the filter limitation in the admin works.

4. Checklist

• I have written tests and verified that they fail without my change
• I have squashed any insignificant commits
• I have created a changelog file with all necessary information about my changes
• I have written or adjusted the documentation according to my changes
• This change has comments for package types, values, functions, and non-obvious lines of code
• I have read the contribution requirements and fulfil them.

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[Soner (shyim)]

Thanks for your Contribution!

[Joshua Behrens (JoshuaBehrens)]

Thank you for the merge 😍 💙