Use normalisation for keyword fields

An elasticsearch feature we could be using: using a [normaliser](https://www.elastic.co/guide/en/elasticsearch/reference/8.5/normalizer.html) allows you to standardise fields in the index. For example, using `lowercase` normalisation will mean that the values `Bla` and `bla` are considered identical for search, aggregations or filtering.

Advantages:
- Keyword fields become more user-friendly to search. E.g. in the parliament corpora, users can search `'pvda'` in the party field, and get documents with `'PvdA'` as the party.
- Puts less pressure on data-cleanup during source extraction.