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 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together