Fix #2922 - Load stylesheet from "custom.css" entrypoint when present (#3332)

* Fix #2922 - Load stylesheet from "custom.css" entrypoint when present

This is pretty much the same way it worked as before, albeit with
having to create app/javascript/packs/custom.js with
require('../styles/custom.scss') (or whatever you want really), which
will be a blank slate for you to import whatever you want

* Remove old assets directory

* Extract font-awesome into common.css and always load it

config/webpack/shared.js

@@ -53,6 +53,12 @@ module.exports = {
           // be loaded together
           return false;
         }
+
+        if (module.resource && /node_modules\/font-awesome/.test(module.resource)) {
+          // extract vendor css into common module
+          return true;
+        }
+
         return count >= 2;
       },
     }),