Add a button which allows editors to state explicitly that a release should not have any cover art.

For example, the editor clicks the "Add Cover Art" button, then instead of selecting a file as usual, a checkbox is ticked to say "No Artwork".

This setting would override any cover art included as a side-effect of a relationship (eg. Amazon ASIN), but would be automatically unset if a user attached a cover art image or specifically linked to cover art and the edit was successfully applied.

Any editor uploading new cover art would have to untick the "No Cover Artwork" box when adding new artwork.