Really fix movable panels inside tab panels by changing the JX.Request serializer
This is weird and almost certainly never intended/desiarable. In particular, it causes a bug where panels embedded inside tab panels are incorrectly draggable.
It's possible this breaks something which relied on the buggy behavior, but that seems unlikely.
Test Plan: Tried to drag a panel inside a tab panel, it really truly didn't work.
Reviewed By: amckinley
Maniphest Tasks: T13272
Differential Revision: https://secure.phabricator.com/D20476