Implement customized sidebar

What does this MR do?

This is basically https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16241 and adding tests and cleaning up old tests.

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

Should we add documentation?

Screenshot

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #14995 (closed)