Consider moving schema validation to pydantic

[jmaupetit]

Purpose

Pydantic seems to be the de facto solution for API/schema validation. As we already use it in the openfun/ralph project, it would make sense to use it in this project. It will offer more pythonic object schema declaration and a python-centric serialization/deserialization.

Proposal

Move all JSON Schema declarations of marion and howard apps to pydantic.