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/mastodon/utils/config.js

@@ -0,0 +1,10 @@
+import ready from '../ready';
+
+export let assetHost = '';
+
+ready(() => {
+  const cdnHost = document.querySelector('meta[name=cdn-host]');
+  if (cdnHost) {
+    assetHost = cdnHost.content || '';
+  }
+});