Use fragments in Epic Tree query and include `__typename`

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/javascripts/related_items_tree/queries/child_items.graphql to use fragments and reduce main query size.

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

#11691 (closed)