Ensure RetrieveElement is called from the DetailsPanel for classifications

Closes #803 (closed)

This is the same bug as I encountered in !1058 (merged); some elements can already be in the store, but have been loaded by the children tree from ListElementChildren. This endpoint does not have the same attributes as RetrieveElement: it has best_classes, worker_version_id and has_children, where RetrieveElement has classifications, metadata_count, creator, rights, thumbnail_put_url and worker_version. I had updated a watcher to ensure the element is reloaded if it is missing its rights; this MR just adds another safeguard to ensure we also reload for classifications.