LP1760893 Add Holdings Part dropdown should list items in order

LP1760893 Add Holdings Part dropdown should list items in order

Without a defined sort, the Part dropdown when editing a holding simply
displays in database order.  Let's be more deterministic about it.

In this case, while slightly surprising, consensus opinion was that
reverse order on label_sortkey provides the best overall usability.
This is obviously true on records with many parts, but can help in all
cases.

Signed-off-by: Dan Pearl <dpearl@cwmars.org>
Signed-off-by: Dan Wells <dbw2@calvin.edu>