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 minor leftovers with Celerity.

Test Plan:
  - Dragged the first item above itself; now an invalid operation with no ghost.
  - Dragged another item to the first position then back to its original position; ghost vanishes.
  - Clean lint.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1344

Differential Revision: https://secure.phabricator.com/D7939