feat: new Web UI build system

- use generator-gulp-angular by @swiip
- remove old static file

webui/src/app/index.constants.js

@@ -0,0 +1,9 @@
+/* global moment:false */
+(function() {
+  'use strict';
+
+  angular
+    .module('traefik')
+    .constant('moment', moment);
+
+})();