Bug 25728: Create AV when adding a new item

Bug 25728: Create AV when adding a new item

We do a bit of refactoring to make the code reusable.

Test plan:
Same as the first patch but when adding/editing an item

QA note: There is a warning from the QA tools
 FAIL   koha-tmpl/intranet-tmpl/prog/en/modules/cataloguing/additem.tt
   FAIL   js_in_body
                A <script> tag found inside head, must be moved to the body (see bug 17858)
I don't think how we could avoid it.

Sponsored-by: Orex Digital
Signed-off-by: Hugo Agud <hagud@orex.es>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>