DS-2163: Multilingual support for browse indexes and discovery facets

Related JIRA issue: https://jira.duraspace.org/browse/DS-2163

Support multilanguage metadata values in the SOLR index and display facets based on the current language of the repository. 

Features:
1) enable or disable feature per facet or browse index 
2) provide in dspace.cfg file the mapping between the repository supported languages and the metadata field languages 
3) multilanguage indexing support for value-pairs metadata fields

Demo: http://esyp.saas.ekt.gr/esyp/?locale=en (try to change the repository language)