Align PublicBody search between views and API

[noeddl]

Search is currently handled differently in regular views and API views, which leads to inconsistent search results depending on whether a search is performed via the search form or through the API.

This is especially the case for PublicBody (see https://github.com/fragdenstaat/issues/issues/293). In the API view, get_searchqueryset is overridden with a custom search and filtering configuration. As a result, recent changes to the search analyzers are not applied there. Using PublicBodyFilterSet instead, as in the view, could resolve this.

However, I noticed several differences between PublicBody search in views and in the API (which is also why the tests I added are currently failing):

• The API uses multi_match search, while the rest of the application appears to use simple_query_string.
• The API filters by IDs, whereas PublicBodyFilterSet uses slugs (which seems to be consistent with other API endpoints, e.g. for FoiRequest).
• The category filter is named categories in the API but category in PublicBodyFilterSet.
• Filters for regions and regions_kind exist in the API, but not in PublicBodyFilterSet.

I’m unsure how to resolve these differences without breaking the API, and which parts are still intentional vs. legacy behaviour.

@stefanw Could you take a look?