Create Schemas (json/php) to describe course elements, modules, etc

The schema files will contain a data architecture for settings and content elements (#522) . This will be used to describe content architecture as well as a configuration for installation and registration.

The schema will contain meta information (like rest api support, Human readable Labels for keys, etc).

The schema is also an abstraction of the data models for courses, lessons, quizzes, etc and can be used along with traits and interfaces to model each element (#597)

This schema will be used to create flexible and extensible forms & interfaces and during data fetches/loads (#575, #567).

The schema can also be used to build an extremely flexible Merge Code API (#564, #560) for personalising every course element.