R1:7ffddcd136a9

Fix two bugs with DraggableList

Summary:
Ref T1344. This fixes two issues with DraggableList:

- In lists which allowed it, you could drag the top item above itself and get a dashed-border ghost item. This didn't make sense and didn't behave well. Just don't treat this operation as valid.
- In lists which allowed it, you could drag any non-top item to the topmost position, then drag it to an invalid position. The dashed-border ghost item would not be removed properly if this happend.
- Also fix some…