Popular Items should no longer have duplicates in the returned list (#21602)

* Popular Items should no longer have duplicates in the returned list

* Introduce concept of model precedence for popular_items

This might also be handled inside the score function, to allow for recency or viewcount to push lower-precedence items
higher in the results, but for now, grouping fetched items by their model and sorting by precedence (dashboard, then
cards, then models, then tables) should be ok.