Should prevent changing visibility if item is published

[pyphilia]

If the item has already been published, it is still possible to toggle the visibility to another setting (ie. pseudonymized).
It doesn't lead to strange behavior because the backend always check if the item is public. But it's a bad user experience.

[spaenleh]

@pyphilia So the backend does not permit to change the visibility setting ? And we should reflect this in the UI ?