Epic -> Issue: Remove the epic link for all issues linked to that epic
Issue -> Epic: Remove epic link if was previously part of an Epic
Issue -> Subtask: Remove epic link to Epic if was linked
Subtask -> Issue: If old issue parent had epic then link issue to epic (to be discussed if we really want this), unless the new issue type is Epic

We shouldn't have to clean up epic label and epic color fields as these are only applicable to epics and therefore will automatically be cleared.

Testing Notes

Q: Make sure this plays well with the change to Epic Name field.
A: The two are completely unrelated

Q: What if subtasks are disabled either before or after trying to convert?
A: Can't disable sub-tasks while there are sub-tasks in the system.

Q: Don't forget to add the Epic Link to the subtask if the parent has one.
A: Epics are always only linked to parent issues, not to subtasks