Bug 33568: Item groups

Bug 33568: Item groups

FIXME - we need tests, but I'd like to make sure there is no an easier
way to do this many to many things. Tomas?
Filter on the "Item group" column needs to be tested!

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Laurence Rault <laurence.rault@biblibre.com>
Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Tomás Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>