Resolve "Improve interactivity of collapsed sidebars"

This MR was merged into https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17714

What does this MR do?

• Makes collapsed sidebar toggle a 60px square with a 1px gray bottom border
• Collapsed sidebar blocks have a gray background on hover

Why was this MR needed?

Requested by #44272 (closed).

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by UX
  • 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 #44272 (closed)