Bug 36396: Link facet with authorised value category - ES

Bug 36396: Link facet with authorised value category - ES

This new feature adds the ability to link an ES facet to an authorised
value category, in order to display a description instead of the code.

Test plan:
1. Go to the ES mapping admin page
2. Notice the new "Authorized value category" column in the facets
   config table
3. Create a new facet that is linked to a search field containing values
   from an authorised value category. Pick this category in the list
4. Save
5. Reindex
6. Notice that the descriptions are now displayed (staff and OPAC) in
   the search result page.

QA: See the related bug report for possible improvements. It's not
easy at all to deal with the "specials" here, a lot of changes must be
done in how we fetch/cache the AVs.

Sponsored-by: The Research University in the Helmholtz Association (KIT)
Signed-off-by: Lukas Koszyk <lukasz.koszyk@kit.edu>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>