Fix some off-by-one issues when drag-and-dropping dashboard panels

Summary: Fixes T5321. There were a couple of off-by-one issues here which could result in inserts into the wrong position.

Test Plan:
  - Dragged panels to the top, bottom, and first position of columns.
  - Dragged panels from one column to another.
  - Reloaded the page after drags, things stayed where I put them.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5321

Differential Revision: https://secure.phabricator.com/D9573