need custom formfield so model can be edited in admin

Currently if you have a `JSONField` in the model you can paste your json into that field and save a new instance of the model.

But when you want to edit that instance you will get a string representation of the python object from `to_python` rather than the json string of your field.  Then when you try to save the model instance after changing some other field it will fail.

I think this is the minimum for a custom field that can handle an edit round-trip.