Bug 8989 - Availability facet needs an id

Bug 8989 - Availability facet needs an id

This patch adds an id to the <li> which contains
the "limit to available" facet in the OPAC and
staff client.

To test, add custom CSS for the staff client and
OPAC to hide the id "availability_facet" and
confirm that the link is no longer visible.

Signed-off-by: Gaetan Boisson <gaetan.boisson@biblibre.com>
Passed-QA-by: Mason James <mtj@kohaaloha.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>