admin: tree selection now expands lazily loaded branches

- the JS item template was rendered without the children container
  because the '__expandable__' placeholder never equaled 'true', so
  children of items created via AJAX were appended into an empty jQuery
  set and never reached the DOM
- affected every branch not pre-rendered on the server, e.g. a third
  level category on product detail when the product is not assigned in
  that branch

Author: vitek-rostislav

assets/js/admin/components/TreeSelectionFormItem.js

@@ -141,6 +141,9 @@ export default class TreeSelectionFormItem {
 
         $newItem.data('load-url', itemData.loadUrl);
         $newItem.data('expandable', itemData.isExpandable);
+        if (!itemData.isExpandable) {
+            $newItem.find('.js-tree-selection-form-children-container:first').remove();
+        }
         if (itemData.isVisible === false) {
             $newItem.addClass($form.data('hidden-item-class'));
         }