Clean project setup creates App.js instead of App.jsx

We changed the file extension from `js` to `jsx` in the webpack config. But the project template for new projects creates files still with `js` extensions.

```
$ react-scripts start
Could not find a required file.
  Name: index.jsx
  Searched in: /Users/richard/workspace/picter/app-graphql-relay/src
error Command failed with exit code 1.
```