Allow mixing of non-JC pages in the same namespace

* If you set nsname=false in configs, you can now share
the namespace between JC-controlled and non-JC pages.
* Fix: 'model'=>null now works in configs
* Fix: null model automatically use default JCContent,
without requiring $wgJsonConfigModels['JsonConfig'] definition.

* This configuration might be too ugly and may need
to be changed, e.g. "allowNsSharing"=true, or possibly
do it automagically - if namespace is defined already,
share it, if not - fully own it.  TBD, and won't affect
this patch much.

Change-Id: Ia912486c693cba2b46403d42017adba26bf8a180
Merge "Allow mixing of non-JC pages in the same namespace"