Reorder CSS to prevent hitting compound selector limit

What does this MR do?

Note: This MR adds a feature to Epic Tree (added in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10999) which is currently behind feature flag :epic_tree.

This MR updates ee/app/assets/stylesheets/components/related_items_tree.scss such that it doesn't trigger selector-max-compound-selectors warnings for complex selectors.

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides

Closes #11691 (closed)