Make Epics and Issues count dynamic in Epic tree

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 makes Epic and Issues count shown in Epic Tree header to be dynamic, where as user continue to expand nodes, the count will update to reflect actual Epics and Issues present in child items. Also, when an item is added or removed to and from the tree, the count will reflect it. Please note that we don't decrease the count when a node is collapsed after expanded since those items are still present in the tree.

Screenshot

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

#11691 (closed)