Localized application search & docs

- [ ] document which keywords (`state:`, `applicant:`) etc. are supported
- [ ] in swagger docs (points to `search.md` currently)
- [ ] in UI (directly, but could point to separate manual for even more details)
- [ ] in `search.md` (should be mostly done for the old values but just need to sync)
- [ ] support multiple forms for each keyword (like `state:`)
- [ ] localized in all the configured languages (e.g. `tila:` in finnish)
- these should match the table columns titles in the UI
- this should also be mentioned in all the docs
- [ ] "API-style" keyword that matches the API data model (like `application/state:`)
- [ ] "API-style" value that matches API data model (like `application.state/closed`)
- [ ] API usage examples somewhere (`search.md`?)