Automatically set new, moved, continued task disposition based on new parent story disposition

1. When creating new task in a story, the task disposition should take the value of the parent story.
2a. When a task is moved to a story of a different future/non active iteration, the task should take the disposition of the target story.
2b When a task is moved to a story of an already started iteration, the task should take the disposition of Added .
3. When a task is moved to another story of the same iteration as the current parent story, the disposition should not change.
4. When a story is moved/continued, its tasks should take the target story disposition

Also make sure this test passes:
   1 Add a story a started iteration, story disposition is 'Added'
   1 Add task to the added story, task disposition is 'Discovered'
   1 Move this story to the future iteration
   1 Story disposition should be 'Planned'
   1 Task disposition should be 'Planned'