R1:aba7d1406ac8

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…