Create-react-app: Add `babel-plugin-transform-class-properties` to babel

[piu130]

If I try to use this tool together with react-native-web several libraries don't work, because in react-native it's common to use syntax like this:

class Test extends Component {
  handleBackPress = () => {
    this.goBack();
    return true;
  }
}

So we should add the babel-plugin-transform-class-properties plugin to babel.

[gaearon]

It's already enabled for application code. We don't plan to enable unstable proposals for library code. It creates a ton of churn in the ecosystem when those proposals are dropped or change semantics.