Bug 39570: Add item form includes itemnumber while adding a new item

Because the template uses the wrong op, add_item rather than cud-additem, to
decide whether to include a hidden input with the itemnumber, if you edit an
item and then add another item you wind up submitting a form that includes the
previous itemnumber when you add a new one. Nothing happens, because the code
for adding an item doesn't look at $input->param('itemnumber'), but since it
doesn't mean anything we shouldn't be submitting it.

Test plan:
1. Without the patch, find any biblio with at least one item attached, and
   in the items table click Edit item on one of them
2. Without needing to making any changes, click Save changes
3. In the page that loads, scroll down to the bottom of the Add item form,
   right-click the Add item button, and choose Inspect
4. A few lines up, above a '<fieldset class="action">,' there will be an
   '<input type="hidden" name="itemnumber" value="633">' with the itemnumber
   of the item you previously edited
5. Apply patch
6. Scroll up, click Actions - Edit on any item, then Save changes
7. Again, right-click the Add item button, Inspect, note that there isn't a
   itemnumber input with the number of the item you previously edited

Sponsored-by: Chetco Community Public Library
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>

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

koha-tmpl/intranet-tmpl/prog/en/modules/cataloguing/additem.tt

@@ -297,7 +297,7 @@
                         [% END %]
 
                         <fieldset class="rows"> [% PROCESS subfields_for_item subfields => subfields %] </fieldset>
-                        [% IF op != 'add_item' %]
+                        [% IF op != 'cud-additem' %]
                             <input type="hidden" name="itemnumber" value="[% itemnumber | html %]" />
                         [% END %]