Compare codeclimate artifacts on the merge request page

Continue of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11379

What does this MR do?

Show how code quality changes by the merge request widget if project has codeclimate job in .gitlab-ci.yml.

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

JS code

Why was this MR needed?

Show how code quality changes by the merge request allow us and other GitLab users to increase software quality they develop.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added (because feature is frontend-based)
• 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?

https://gitlab.com/gitlab-org/gitlab-ce/issues/4044