Ee 44272 improve interactivity of sidebar

This MR was merged into !5212 (merged)

What does this MR do?

Backport of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18135

• Makes collapsed sidebar toggle a 60px square with a 1px gray bottom border

Why was this MR needed?

Requested by https://gitlab.com/gitlab-org/gitlab-ce/issues/44272.

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/44272.