Multilang: use indexed fields in client queries

[etj]

According to GeoNode/geonode#13644, the backend now supports text search based on database text indexes.

The client at the moment uses as query params search_field+search to search some text in the given fields.
The client should now use one search index using the params search_index+search, where the default indexes are title, title_abstract, all, defined in this way:

METADATA_INDEXES = {
    "title": ["title"],
    "title_abstract": ["title", "abstract"],
    "all": ["title", "abstract", "supplemental_information"],
}

Note that the old search search_field+search will still be supported.