Add isInsurance endpoint

[Johennes]

Changes:

• New endpoint /v1/domain/isInsurance
• Changed "domain" to "server" / "serverName" (avoids misunderstandings, e.g. about subdomains)
• Removed operationIds (seemed unused)
• Shortened and deduplicated some of the summaries and descriptions
• Removed default status codes from all responses (not an actual status code, response schema already inherited)
• Pushed parameters one level down beneath get (so that we don't accidentally reuse them in future)
• Wrapped findByIk response in object (matching isInsurance, more idiomatic and extensible)
• Translated German descriptions in InfoObject