Bug 26664: Fix sorting for date last seen in inventory tool

Bug 26664: Fix sorting for date last seen in inventory tool

This adds the standard datatable sorting for dates to the inventory
table as described here:
https://wiki.koha-community.org/wiki/DataTables_HowTo#Sorting_dates_regardless_of_date_format_preference

To test:
- Go to the inventory tool
- Search for a big sample list of items with different date last seen
- Sort the table now by "date last seen" by clicking on the table header cell
- Try different settings of the DateFormat system preferences
- Verify sorting is incorrect (alphabetic, not by date)
- Apply patch
- Repeat testing and very all is sorting correctly now

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>