Finish API for saved scenario users

[noracato]

After the multi user project (#4226) the API to add users to saved scenarios remained unfinished.
I left the code in there so we could one day finish it

[github-actions]

This issue has had no activity for 60 days and will be closed in 7 days. Removing the "Stale" label or posting a comment will prevent it from being closed automatically. You can also add the "Pinned" label to ensure it isn't marked as stale in the future.