Add es6 gem

## What does this MR do?
Add ES6 to ES5 compiling via Babel and the sprockets es6 gem.

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

## Why was this MR needed?
So we can write ES6

## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/issues/20098

## Screenshots (if relevant)
![Screen_Shot_2016-07-25_at_10.56.02_AM](/uploads/04ceeedb8235fd3a7601bc4bece313a8/Screen_Shot_2016-07-25_at_10.56.02_AM.png)

![Screen_Shot_2016-07-25_at_10.55.54_AM](/uploads/163c46a3d878924ab1717fccca468003/Screen_Shot_2016-07-25_at_10.55.54_AM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [x] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5473