Make promoted epics a child of the original issue's epic

When using the /promote quick-action, if the issue has an epic assigned to it already, we should consider making the new epic a child of this existing epic instead of it being completely separate.

Example:

Issue A has Epic A assigned. If I /promote the issue, Issue A gets closed and Epic B gets created. I'm proposing that Epic B created by this quick-action be a child-epic of Epic A.