Refactor away project_users_with_descendants method

What does this MR do and why?

Related issue: #417455 (closed)

Refactor away project_users_with_descendants method

As project_users_with_descendants has a cross-join we do not want further usages.

We collapse project_users_with_descendants into its only remaining caller.

Also, update the URLS for allow_cross_joins_across_databases

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