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