Resolve "Elasticsearch: Highlighting in the code results does not work"

What does this MR do?

Fixes the Elasticsearch results highlighting code path and adds a tests to avoid future regressions.

EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10420

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

Why was this MR needed?

Code highlighting is broken in search results at the moment

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• 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?

Closes #1567 (closed)