LP#1504615: include part names when sorting copies for public catalog display

LP#1504615: include part names when sorting copies for public catalog display

This adds a level of sorting to allow certain parts to be more
easily found in the list.

The hierarchy will now be:
Org Unit / Call Number / Part Label Sortkey / Copy Number / Barcode

Previously, the hierarchy was
Org Unit / Call Number / Copy Number / Barcode

To test:

[1] Create a record with several copies under one volume
    using different monograph parts.  Observe that the
    order of copies in the record details page in the
    public catalog is not sorted by part.
[2] Apply the patch and view the records details page; this
    time, the copies should be sorted by part.

Signed-off-by: Dan Pearl <dpearl@cwmars.org>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>