Add the assets source

core/admin/assets/app.js

@@ -0,0 +1,8 @@
+import 'select2';
+
+jQuery("document").ready(function() {
+    jQuery(".mailselect").select2({
+        tags: true,
+        tokenSeparators: [',', ' ']
+    })
+});

core/admin/assets/vendor.js

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+// jQuery
+import jQuery from 'jquery';
+import 'select2/dist/css/select2.css';
+
+// bootstrap
+import 'bootstrap/less/bootstrap.less';
+import 'bootstrap';
+
+// FA
+import 'font-awesome/scss/font-awesome.scss';
+
+// AdminLTE
+import 'admin-lte/build/less/AdminLTE-without-plugins.less';
+import 'admin-lte/build/less/select2.less';
+import 'admin-lte/build/less/skins/skin-blue.less';
+import 'admin-lte/build/js/Layout.js';
+import 'admin-lte/build/js/ControlSidebar.js';
+import 'admin-lte/build/js/PushMenu.js';
+import 'admin-lte/build/js/BoxRefresh.js';