Bug 26860: facets.inc refactor availability facet

Bug 26860: facets.inc refactor availability facet

To prepare new facet, this patch refactors the availability facet,
without any change in restult.
Appart from an id 'facet-availability', like other facets.

Test plan:
1) Perform a search in staff interface
2) Check availability facet displays well
3) Click on it
4) Check availability facet displays well

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>