Bug 18235: ES - Facets configurable

Bug 18235: ES - Facets configurable

This patch adds a new section 'Facet order' in the Biblio tab of the
'Search engine configuration' admin page of the Elastic mappings.
The idea is to let the librarians define the facet to display and order
them as their needs.

The ergonomic is not perfect and I am open to any suggestions.

Test plan:
Move up and down the field list to order the facets
Hide/show some facets
Rebuild index
At the OPAC and the staff interface you should see the changes on the
search result page.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>

Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>