Split XML serialization and validation contracts.

@gradyward 
@wedaly 

Covers: 
* XML serialization changes
* Validation tweak (minor)
* Studio Mixin update
* Buncha tests

Continues to support reading in full XML configurations (for workbench, etc) but also allows serialization and a more granular level. 

Mixin now adheres to a JSON update contract rather than purely XML. 
This seems to have lost the `strict_post_release=False` kwarg -- is that intentional?  If I remember correctly, there was an issue with importing a problem that overwrites something that's already been released.