#2714 prepare to define full day zero schema

[iamleeg]

In this branch:

• all document validation is made generic (i.e. no field names/types are assumed in the validation logic) and pushed up to the Document class.
• all fields are defined using the Field model, including those on the day zero schema.
• DayZeroCase class is removed as these fields can be added directly to the Case class.
• JSON file defining the day zero data dictionary.

With these changes in place, we can easily implement the day zero data dictionary by editing the JSON file.