search: improve default behaviour/accuracy

[slint]

We should have a look at search performance across all vocabularies and tune them all at the same time:

• Review all text/keyword fields and add sub-fields with the reverse type to allowing for more precise queries (or possible aggregations/facets)
• Have a look at what fields are used for free-text search, using the settings.index.query.default_field

[wgresshoff]

We made some advances regarding suggestions covering multilingual fields (which are stored as Json's). We added (e.g. in contrib/subjects/config.py):

        fields=[  # suggest fields
            "subject.*^100",
            "synonyms^20",
        ],

Now the contents in every language are suggested.