Merge branch 'users-es6' into 'master'

Front End - Rewrote ES5 `user.js` to ES6 `user.js.es6`

## What does this MR do?

This MR removes the `users.js` file in `app/assets/javascripts/users.js` and replaces it with ES6.

* It also adds the file extension `.es6` so that the sprockets babel gem can transpile it to ES5.
* The new file is: `users.js.es6` in `app/assets/javascripts/`
* Replaced `new User` with `new gl.User` in `app/views/users/show.html.haml`

## Are there points in the code the reviewer needs to double check?

I followed the AirBnb guide, but if there are any mistakes let me know!

## Why was this MR needed?

Because the entire coffee script codebase was transferred over to ES5 with a plan to move everything over to ES6 😄 

## What are the relevant issue numbers?

#20098 

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5676

app/assets/javascripts/user.js → app/assets/javascripts/user.js.es6

-(function() {
-  this.User = (function() {
-    function User(opts) {
+(global => {
+  global.User = class {
+    constructor(opts) {
       this.opts = opts;
-      $('.profile-groups-avatars').tooltip({
-        "placement": "top"
-      });
+      this.placeProfileAvatarsToTop();
       this.initTabs();
-      $('.hide-project-limit-message').on('click', function(e) {
-        $.cookie('hide_project_limit_message', 'false', {
-          path: gon.relative_url_root || '/'
-        });
-        $(this).parents('.project-limit-message').remove();
-        return e.preventDefault();
+      this.hideProjectLimitMessage();
+    }
+
+    placeProfileAvatarsToTop() {
+      $('.profile-groups-avatars').tooltip({
+        placement: 'top'
       });
     }
 
-    User.prototype.initTabs = function() {
+    initTabs() {
       return new UserTabs({
         parentEl: '.user-profile',
         action: this.opts.action
       });
-    };
-
-    return User;
-
-  })();
+    }
 
-}).call(this);
+    hideProjectLimitMessage() {
+      $('.hide-project-limit-message').on('click', e => {
+        e.preventDefault();
+        const path = gon.relative_url_root || '/';
+        $.cookie('hide_project_limit_message', 'false', {
+          path: path
+        });
+        $(this).parents('.project-limit-message').remove();
+      });
+    }
+  }
+})(window.gl || (window.gl = {}));

app/views/users/show.html.haml

@@ -123,6 +123,6 @@
 :javascript
   var userProfile;
 
-  userProfile = new User({
+  userProfile = new gl.User({
     action: "#{controller.action_name}"
   });