Create a sub-Task column that displays issue's sub-tasks (if any) in Issue Navigator

The column should display the issue keys of subtasks.
The issue keys should be striked through if the issue has been closed (resolved)
The column should be sortable, such that issues with all resolved subtasks appear together (i.e. apart from an issue with at least some unresolved subtasks).

Note that the number of sub-tasks an issue has, does not matter for sorting. That is, if the issue has one sub-task unresolved it is treated the same as an issue which has 5 unresolved sub-tasks.

The issues with all unresolved sub-tasks will appear before issues with all resolved subtasks, followed by issues with no sub-tasks in Ascending (default) sort.