JSONFormField, used by JSONField, knew how to take JSON data and

serialize it for use in the field, but it didn't then deserialize that

data back after cleaning the field. This didn't always matter in all

cases, but depending on how the form was constructed and validated, it

could result in validation errors or incorrect data being made available

to callers.
This change implements the to_python method, which handles the

deserialization side. This should ensure that the resulting object is

always going to be valid JSON data (or will result in a

ValidationError).

Unit tests pass, including new ones in Review Board that broke prior to

this fix.