Update JSON webservice to use JsonDocument instead of deprecated JsonObject

This changes the output format of the JSON webservice. Results are no longer
wrapped (twice) in an object. For example, the request

    /checkSyntax?variant=AB026906.1:c.274del

used to return

    {"checkSyntaxResponse": {"checkSyntaxResult":
      {"valid": true, "messages": {"SoapMessage": []}}}}

and now returns

    {"valid": true, "messages": []}

We also use "soft" input validation.