Bug 35657: Add option to set assignee during update

Bug 35657: Add option to set assignee during update

This patch exposes the UI to allow setting assignee from the ticket
update modal.

Test plan
1) Apply the patches and run the database updates
2) Enable catalog concerns - `CatalogConcerns` or `OPACCatalogConcerns`
3) Submit a catalog concern via the OPAC or Staff client biblio page
4) Navigate to the concerns management page 'Cataloging > Catalog
   concerns'
5) Note there is no assignee displayed in the status field for your new
   concern.
6) Click the concern to view it's details
7) Note that in the modal there is now a new 'Change assignee' option
8) Use this new option to search for and assign a librarian to the
   concern.
9) Submit the update
10) Your assignee should now appear in the 'status' data field in the
    table
11) Clicking through to details again, you should see when the assignee
    was set and should also be able to re-assign it

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