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)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • 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