Change how theme style packs are loaded

Load the `common` style pack, and then charge the style pack for the current
skin, independent from any selected JS pack.

app/javascript/packs/common.js

@@ -1,5 +1,5 @@
+/* This file is a hack to have something more reliable than the upstream `common` tag
+  that is implicitly generated as the common chunk through webpack's `splitChunks` config */
+
 import './public-path';
 import 'font-awesome/css/font-awesome.css';
-import 'styles/application.scss';
-
-require.context('../images/', true);