Add setting for secondary sorting on Best Seller

There are currently system settings which allow the sort order of items to be based on "Best Seller." That ranking is driven by the value stored in the External Ranking custom field. When there are two items with the same exact External Rank value, my understanding is that the sorting is based on an internal UID of that item's place in the Solar db. It would be better to have a secondary rank value so our customers can indicate another value these items with equal external ranks should be sorted by.