Add missing search_field_core to corpus definitions

[Meesch]

Several corpora do not have the searchFieldCore parameter for any field configured in the corpus definition, leading to empty lists when a user selects the gear icon next to the search field/download button. See also this screenshot:

Corpora without searchFieldCore:

• DIOPTRA-L
• Judicial System Netherlands

[lukavdplas]

Perhaps fields with display_type = 'text_content' should have 'search_field_core' = true by default.

[lukavdplas]

I would also not be opposed to just removing the search_field_core parameter altogether. Just show all text fields.

Older corpora sometimes have keyword fields that can only be queried with text search, which are often the once not making the "core" cut. But since keyword fields don't support full-text search, they should not be included in this list anyway imo.