refactor UserTabs class to ES module syntax

app/assets/javascripts/main.js

@@ -156,7 +156,6 @@ import './todos';
 import './tree';
 import './usage_ping';
 import './user';
-import './user_tabs';
 
 // eslint-disable-next-line global-require, import/no-commonjs
 if (process.env.NODE_ENV !== 'production') require('./test_utils/');

app/assets/javascripts/user.js

 /* eslint-disable class-methods-use-this, comma-dangle, arrow-parens, no-param-reassign */
 
 import Cookies from 'js-cookie';
+import UserTabs from './user_tabs';
 
 class User {
   constructor({ action }) {
@@ -17,7 +18,7 @@ class User {
   }
 
   initTabs() {
-    return new window.gl.UserTabs({
+    return new UserTabs({
       parentEl: '.user-profile',
       action: this.action
     });

app/assets/javascripts/user_tabs.js

@@ -60,7 +60,7 @@ content on the Users#show page.
    </div>
 */
 
-class UserTabs {
+export default class UserTabs {
   constructor ({ defaultAction, action, parentEl }) {
     this.loaded = {};
     this.defaultAction = defaultAction || 'activity';
@@ -171,6 +171,3 @@ class UserTabs {
     return this.$parentEl.find('.nav-links .active a').data('action');
   }
 }
-
-window.gl = window.gl || {};
-window.gl.UserTabs = UserTabs;