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

app/helpers/style_helper.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+module StyleHelper
+  def stylesheet_for_layout
+    if asset_exist? 'custom.css'
+      'custom'
+    else
+      'application'
+    end
+  end
+
+  def asset_exist?(path)
+    true if Webpacker::Manifest.lookup(path)
+  rescue Webpacker::FileLoader::NotFoundError
+    false
+  end
+end