Users wanted a feature where they could specify
which users can create public streams and which users can
create private streams.
This splits stream creation code into two parts,
public and private stream creation.
Fixes#17009.
This change doesn't have proper test coverage and also doesn't work as
intended, since due to the structure of the function, we'll only
complain about invalid policy values for members in unlikely situations.
This reverts commit c69a968ffe.