Bug 31692: Allow librarians to change hold type in staff client

Bug 31692: Allow librarians to change hold type in staff client

To test:
1. Apply patch
2. Add item level hold to a record/item, make sure patron has no other
   holds on that record
3. Go to /cgi-bin/koha/reserve/request.pl?biblionumber=xxx where xxx is
   the record you placed the hold for
4. Under "Existing holds" table, in "Details" column you should see
   "Only item <barcode>" dropdown
5. Select "Next available" from the dropdown
6. Click Update hold(s)
7. Observe dropdown is gone and cell value has changed from
   "Only item <barcode>" to "Next available"
8. Cancel the hold and add two item level holds for the same patron
9. Under "Existing holds" table, in "Details" column you should see
   "Only item <barcode>", but no select dropdown

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>