message: Add classnames package
diff: "@types/node": "12.0.10",
  "@types/react": "16.8.22",
  "@types/react-dom": "16.8.4",
  + "classnames": "^2.2.6",
  "react": "^16.8.6",
  "react-dom": "^16.8.6",
  "react-scripts": "3.0.1",
  