Bug 28831: Database revision, add new preference

Bug 28831: Database revision, add new preference

The preference OPACResultsUnavailableGroupingBy allows you to
group unavailable items by substatus, only showing item counts, on
the OPAC XSLT results.

This is meant to be useful for larger consortia.

Test plan:
Run dbrev.
Check new pref on Admin, preferences, OPAC tab.

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>