lp1182605 Abstracting out common functionality from code that sorts call numbers...

lp1182605 Abstracting out common functionality from code that sorts call numbers by label_sortkey

Code had been added to insure that LC call numbers are sorting as they should by asset.call_number.label_sortkey.
Code was duplicated in the 'call_number column' in the 'columns' group and in the 'hold_columns'group.

I abstracted the logic out of 'sort_value' and 'render' in the 'call_number' column in the 'columns' group and from
'render' in the 'call_number' column in the 'hold_columns' group to a utility function  called
'sort_call_numbers_by_label_sortkey'.

Signed-off-by: Victoria Lewis <vcamklewis@gmail.com>
modified:   xul/staff_client/server/circ/util.js