Applying CE Patch for global deferred JS loading

What does this MR do?

It will activate deferred Javascript loading globally gitlab-org/gitlab-ce!12759

Why was this MR needed?

CE -> EE Patch

Does this MR meet the acceptance criteria?

• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?