Bug 28831: Add the new group by substatus-block

Bug 28831: Add the new group by substatus-block

Test plan:
Change OPACResultsUnavailableGroupingBy to substatus.
Verify the result on opac results. (You need a few unavailable items
with different statuses on a few branches.)

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>