🐛 Fixed internal css leaking outside for search

refs https://github.com/TryGhost/Team/issues/1675
closes https://github.com/TryGhost/Ghost/issues/15020

- the main css file used for search was accidentally 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

ghost/sodo-search/src/index.js

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