FIX: select-kit was in the wrong place

`app/` means we want to merge it into our application there. `addon`
means give it its own module namespace, which is what we wanted.

app/assets/javascripts/application.js

@@ -1,6 +1,6 @@
 //= require_tree ./discourse-common/addon
 //= require ./polyfills
-//= require_tree ./select-kit/app
+//= require_tree ./select-kit/addon
 //= require ./discourse/app/app
 //= require ./app-boot