message: Make compile with g++-8
diff: @@ -389,7 +389,7 @@ void App::UserAPIKeyProviderClient::fetch_api_keys(std::shared_ptr<SyncUser> use
  try {
  auto api_key_array = std::vector<UserAPIKey>();
  - auto json_array = std::vector<nlohmann::json>(json);
  + auto json_array = json.get<std::vector<nlohmann::json>>();
  for (nlohmann::json& api_key_json : json_array) {
  api_key_array.push_back(
  App::UserAPIKey {
  