Some featured albums not showing correct thumbnail

[fynngodau]

On the Bandcamp featured page, there's sometimes albums that are not available for purchase yet, and can only be preordered. These seem not to show the correct thumbnail.

It's a bit difficult to test because of #584.

[fynngodau]

Either there is another issue specifically with preorder albums that I don't remember, or I was actually looking at some albums in the featured screen that have no art_id, just an item_art_id. Not sure what the difference is supposed to be but we should fall back to the latter if the first is not available.