Convert react-error-overlay to React

Created by: gaearon

It's currently pretty complicated. We should turn this into a separate package with a build step, and probably use React as a dependency instead of vanilla JS code.

We shouldn't have the "duplicate React" issue because it's technically inside an iframe and would get its own copy of React. So if I understand it right, they wouldn't share any state, and if React in the app breaks, the one in the overlay should not be affected.