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