Bug 35774: Use itemnumber instead of biblioitemnumber

Bug 35774: Use itemnumber instead of biblioitemnumber

With Bug 33639 already pushed to master there should be NO change in behavior.

To test:
1. APPLY PATCH
2. Create one or more item groups for a bib if needed
3. Create a new item, and at the bottom of the 'Add item' screen, select the item group to which you want this item added. Save.
4. The item should succesffuly be added to the item group.
5. Add a new item and to the same record and this time try the 'Create a new item group' from the dropdown.
6. Name the new item group.
7. Make sure it is correctly added to the item group.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>