Backport panels.scss styles from EE

What does this MR do?

Backport panels.scss styles from EE. De-nest some selectors

See https://gitlab.com/gitlab-org/gitlab-ee/blob/43a570dcf5cbd0a98572d5601e9000e906791db0/app/assets/stylesheets/framework/panels.scss

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

Why was this MR needed?

Avoid future conflicts in panels.scss and keep both files in sync

Does this MR meet the acceptance criteria?

• 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?