Change how CDN_HOST is passed down to make assets build reproducible (#14381)

* Change how CDN_HOST is passed down to make assets build reproducible

* Change webpacker/webpack configuration to dynamically load publicPath based on meta header

* Fix embedded layout missing the cdn-host meta header

app/javascript/packs/admin.js

@@ -1,3 +1,4 @@
+import './public-path';
 import { delegate } from '@rails/ujs';
 import ready from '../mastodon/ready';