Really fix movable panels inside tab panels by changing the JX.Request serializer

Summary:
Depends on D20475. Ref T13272. Currently, if you JX.Request with data like {x: null}, we submit that as ?x=null, i.e. as though null was the string "null".

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.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20476