(WIP) Class diagram transfer

A start at transferring content from the class diagram into code.  Focuses on School/Course/Klass/Educator/Student type models.  

@Dantemss I'd appreciate a once over of this PR if you have a chance.
I would add a null: false here. In general you can do that on anything that has dependent: :destroy on the other end (School has_many :courses, dependent: :destroy)