Bug 9988: Add get_usage_count and linked_biblionumbers to Koha::Authority

Bug 9988: Add get_usage_count and linked_biblionumbers to Koha::Authority

When replacing the Zebra code in sub merge, we actually need CountUsage
as well as the results it gets from SearchEngine.

In order to get count and/or results, we now create:

[1] instance methods get_usage_count and linked_biblionumbers in
    Koha::Authority,
[2] class methods of the same name in Koha::Authorities.

The instance method calls the class method. The class method can be used
for deleted authority records, and in 'legacy calls' before refactoring.

Note: On BZ 18149 we will replace all CountUsage calls.

Test plan:
Run t/db_dependent/Koha/Authorities.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>