Ensure items cannot have more than one pending item curation

[nicosantangelo]

Each published third party item will have an associated ItemCuration created for it. Once it's approved, another item curation might be created to signify changes on the item. But what can't happen is for an item to have +2 ItemCurations in the pending status.

We should ensure it either via code or, more likely, as a restriction in the database