Merge branch 'es6-gitlab-sytanx-highlighting' into 'master'

Added '*.js.es6 gitlab-language=javascript' to .gitattributes

## What does this MR do?

According to the branch name this adds the `sytanx` highlighting for `.js.es6` files, but this actually adds the `syntax` highlighting. 😆 

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

Is this the right place? 🏃 

## Why was this MR needed?

We need pretty ES6 diffs.

## What are the relevant issue numbers?

Flyin' solo 🛫 

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [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
  - [ ] 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 !5486

.gitattributes

 CHANGELOG merge=union
+*.js.es6 gitlab-language=javascript