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/features/emoji/emoji.js

@@ -1,11 +1,10 @@
 import { autoPlayGif } from '../../initial_state';
 import unicodeMapping from './emoji_unicode_mapping_light';
+import { assetHost } from 'mastodon/utils/config';
 import Trie from 'substring-trie';
 
 const trie = new Trie(Object.keys(unicodeMapping));
 
-const assetHost = process.env.CDN_HOST || '';
-
 // Convert to file names from emojis. (For different variation selector emojis)
 const emojiFilenames = (emojis) => {
   return emojis.map(v => unicodeMapping[v].filename);