Bug 18213: Add language facets to Elasticsearch

Bug 18213: Add language facets to Elasticsearch

This patch add language as a facet to ES results - it adds
a new template plugin for languages to get the appropriate
description given an iso 639-2 code

To test:
1 - Make sure you have records with differing languages (in the MARC21 008
        field characters 35-37 or UNIMARC 101a)
2 - Apply patch
3 - Reload Elasticsearch settings:
http://localhost:8081/cgi-bin/koha/admin/searchengine/elasticsearch/mappings.pl?op=reset&i_know_what_i_am_doing=1
4 - Reindex your records
5 - Search for a phrase that will return results in several languages
6 - Verify you see factes correctly labelled for 'Language'
7 - Verify the facets work
8 - Verify both opac and staff results
9 - prove t/db_dependent/Languages.t

Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>