Bug 25553: Make date columns sortable on the edit items table

Bug 25553: Make date columns sortable on the edit items table

When editing items, the table at the top contain several columns that
have date values, but they cannot be sorted by dates correctly.

Test plan:
Have several items with different dates in columns that contain dates,
like items.dateaccessioned, items.datelastseen)
Sort the column and confirm that with this patch the lines are sorted
correctly

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>