Ability to split a story and select which sub-tasks to re-parent

As a GH User, I would like to be able to split a story and select which sub-tasks to re-parent
Original issue raised for (Classic) GreenHopper.

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This is different than cloning in that we want to clone the issue and re-parent sub-tasks not copy them. Maybe Atlassian could put this in their core product but GH the Agile boards are primarily where it would be used.

Acceptance criteria:

1. Should have a way to split an issue from either the Planning or Task Boards Backlog or Active sprints/Kanban board. Could be on the card drop-down menu.
2. Should clone parent issue without sub-tasks.
3. Should allow selection of 0 or many sub-tasks to re-parent to the new issue.