Bug 26866: Sort items table on additem by cn_cort

Bug 26866: Sort items table on additem by cn_cort

This patch changes the data-order attribute for the item callnumber column
to use the item's cn_sort instead of the callnumber value.

Test plan:
1. Create a bib
2. Add three items with source of classification LLC
    call numbers:
    JC43 .G6 1890
    JC330 .F74 2000
    JC480 .R63 2006
4. On additem.pl sor the items table by 'Full call number'
5. Confirm the items are now ordered correctly by cn_sort
6. Confirm the other columns still sort correctly

Signed-off-by: Roman Dolny <roman.dolny@jezuici.pl>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>