Contribution: Impossible to compile and to launch test application

After cloning the repository and running `npm install`. I get the following error:
`react-sortable-tree/website/index.js:5:7: Cannot resolve dependency '../style.css'`

I see that this file exists in `react-sortable-tree/website/sandbox` so I apply the correction in `react-sortable-tree/website/index.js`

Once this correction is done, the application runs but doesn't show anything because any file that is retrieved (even the main javascript) return the `index.html` file