Fix participants css

What does this MR do?

Bring https://gitlab.com/gitlab-org/gitlab-ce/commit/7389bb98167551a850be52e6683ecc93c4734a05 to EE

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

Shouldn't be

Why was this MR needed?

Fix UI

Screenshots (if relevant)

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

closes #2423 (closed)