Cleanup minor UX issues in the performance dashboard

What does this MR do?

Cleans up some other minor UX issues following from this MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12616

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • 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 #34653 (closed)