message: read query params for json in dashboard endpoint
diff: @@ -1953,6 +1953,9 @@ class Superset(BaseSupersetView):
  'common': self.common_bootsrap_payload(),
  }
  + if request.args.get('json') == 'true':
  + return json_success(json.dumps(bootstrap_data))
  +
  return self.render_template(
  'superset/dashboard.html',
  entry='dashboard',
  