Better api.Externalable responses for apps/clients

#55 (feature) - types that implement api.Externalable now return responses if data was sent and saved successfully. This will help apps/clients to use data to build UI or confirm to users that requests were successful or redirect a user to their content.

Example response to a POST /api/content/external?type=Review which also implements api.Trustable

{
  "data": [
    {
        "id": "6", // will be omitted if status is pending
        "type": "Review",
        "status": "public"
    }
  ]
}