Bug 18264 - Course reserves - use itemnumber for editing existing reserve items

Bug 18264 - Course reserves - use itemnumber for editing existing reserve items

This patch alters the crouse reserves module to use itemnumber as the
lookup method for existing reserves as opposed to barcode

To test:
1 - Apply patch
2 - Add some items to a course reserves - all should work as expected
3 - Delete the barcode for an item on reserve
4 - Note the edit link still resolves to the correct item

Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>