🐛 Fixed internal css leaking outside iframe

refs https://github.com/TryGhost/Team/issues/1675

- the main css file was imported in the root js script, causing it to be included alongside the final minified bundle and get injected outside the iframe.
- the main css is pushed as a separate file outside of bundle and used directly as link inside the iframe instead

apps/comments-ui/src/index.js

@@ -1,6 +1,5 @@
 import React from 'react';
 import ReactDOM from 'react-dom';
-import './index.css';
 import App from './App';
 const ROOT_DIV_ID = 'ghost-comments-root';