Bug 22321: Allow managing the borrowernumber tied to ILL request

Bug 22321: Allow managing the borrowernumber tied to ILL request

Sometimes an ILL request is connected to the wrong borrower, and it
would be nice to be able to connect it to the right one.

In Swedish ILL the cardnumber of the patron connected with an ILL
request is entered into the central Libris system and then propagated
to Koha through the Libris API. This can lead to errors, and we have
created a dummy "ILL patron" to catch these. It would then be good
to be able to connect the ILL to the right patron in Koha.

This commit makes it possible to edit illrequests.borrowernumber.

To test:
1. Apply patch
2. Create two patrons, A and B
3. Create a new ILL request for patron A
4. Add a hold for patron A where biblionumber is the biblio tied
   to this ILL request
5. Go to
cgi-bin/koha/ill/ill-requests.pl?method=edit_action&illrequest_id=161
6. Observe Patron ID is editable
7. Change borrowernumber to patron B's borrowernumber
8. Click save
9. Observe ILL request is now for patron B
10. Go to patron B's details at
cgi-bin/koha/members/moremember.pl?borrowernumber=B_borrowernumber
11. At the bottom section of the page, click 1 Hold(s)
12. Observe the hold you placed at step 4 has transferred to
    patron b

Signed-off-by: Magnus Enger <magnus@libriotech.no>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>