Frontend for Merge Request Diff

This merge request improves the UX for the merge request diff feature which was recently implemented here (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6127). Specifically, it styles various parts of the diff feature to match the designs, it disables comment-related buttons in states where comments are disabled, and it adds a 'Show latest version' button for convenience.

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

I could use feedback on this MR's fidelity to the design.

Why was this MR needed?

Neccessary styling improvements for basic UX of this feature, and enabled comment buttons are not functional and thus need to be disabled in certain states.

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

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/21427 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6127

cc: @jschatz1