DEV: select-kit 2 (#7998)

This new iteration of select-kit focuses on following best principales and disallowing mutations inside select-kit components. A best effort has been made to avoid breaking changes, however if you content was a flat array, eg: ["foo", "bar"] You will need to set valueProperty=null and nameProperty=null on the component.

Also almost every component should have an `onChange` handler now to decide what to do with the updated data. **select-kit will not mutate your data by itself anymore**

app/assets/javascripts/admin/routes/admin-user-index.js.es6

@@ -19,7 +19,7 @@ export default DiscourseRoute.extend({
     controller.setProperties({
       originalPrimaryGroupId: model.primary_group_id,
       availableGroups: this._availableGroups,
-      customGroupIdsBuffer: null,
+      customGroupIdsBuffer: model.customGroups.mapBy("id"),
       model
     });
   }