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

@@ -0,0 +1,14 @@
+/* This is needed for the wicg-inert polyfill */
+
+[inert] {
+  pointer-events: none;
+  cursor: default;
+}
+
+[inert],
+[inert] * {
+  user-select: none;
+  -webkit-user-select: none;
+  -moz-user-select: none;
+  -ms-user-select: none;
+}