Bug 28831: Add choose/when, and change indentation

Bug 28831: Add choose/when, and change indentation

As a first step:
[1] Add the new pref OPACResultsUnavailableGroupingBy in the xslt,
    and pass it via C4/XSLT.
[2] Add a choose/when construction in the third part of Availability.
[3] Add indentation (8 spaces) of the inner block handling group by branch.

Test plan:
Make sure that OPACResultsUnavailableGroupingBy eq branch.
Verify that OPAC results are still untouched.

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>