Change template project to functional rather than class-based component. #6558

The template project for create-react-app still uses React.Component in App.js.

Given that hooks are looking to deprecate class-based components, now would seem a good time to turn it into a functional component.

It look like it's a simple enough change. But before I do it, I wanted to make sure that no one else was doing this, and that there wasn't a good reason to keep App.js as class-based.