Allow changing item parent Epic using drag and drop interaction

Summary

With #13633 (closed), we're adding support for reordering Epics and Issues as a combined list. The next logical step in the Epics Tree UX is to allow user to move any item (Issue or an Epic) inside of an Epic, and thus changing parent Epic of that item. This was also discussed to be included in the tree originally but we're breaking down entire effort into smaller issues, so this issue is tracking just the part of nested drag and drop. When dragging an item to a closed child epic, the child epic should expand so they can drop the item within.

Documentation to Update:

https://docs.gitlab.com/ee/user/group/epics/#reordering-issues-and-child-epics

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