Schema validation for student training examples

[ORA-641](https://openedx.atlassian.net/browse/ORA-641): Add schema validation to studio_mixin

This will break the editor for student training until the other subtasks of [ORA-39](https://openedx.atlassian.net/browse/ORA-39) are finished.  Nonetheless, I think it makes sense to merge this now to avoid merge conflicts down the road.

@stephensanchez @gradyward please review.