Use the assets pipeline to load `inert.css` (#28701)

app/javascript/packs/inert.js

@@ -0,0 +1,4 @@
+/* Placeholder file to have `inert.scss` compiled by Webpack
+   This is used by the `wicg-inert` polyfill */
+
+import '../styles/inert.scss';

app/javascript/styles/inert.scss

@@ -1,3 +1,5 @@
+/* This is needed for the wicg-inert polyfill */
+
 [inert] {
   pointer-events: none;
   cursor: default;

app/views/layouts/application.html.haml

@@ -28,12 +28,14 @@
 
     = stylesheet_pack_tag 'common', media: 'all', crossorigin: 'anonymous'
     = stylesheet_pack_tag current_theme, media: 'all', crossorigin: 'anonymous'
+    -# Needed for the wicg-inert polyfill. It needs to be on it's own <style> tag, with this `id`
+    = stylesheet_pack_tag 'inert', media: 'all', id: 'inert-style'
+
     = javascript_pack_tag 'common', crossorigin: 'anonymous'
     = preload_pack_asset "locale/#{I18n.locale}-json.js"
     = csrf_meta_tags unless skip_csrf_meta_tags?
     %meta{ name: 'style-nonce', content: request.content_security_policy_nonce }
 
-    = stylesheet_link_tag '/inert.css', skip_pipeline: true, media: 'all', id: 'inert-style'
     = stylesheet_link_tag custom_css_path, skip_pipeline: true, host: root_url, media: 'all'
 
     = yield :header_tags