Add a class(room) support

[zaratan]

Why ?

Because we want to implement the database we designed

Must have

• [ ] The class migration and model
• [ ] The link between lesson and class
• [ ] The link between a class and a creator (teacher)
• [ ] Controller: normal methods for a LCRUD (with policies)
• [ ] Controller: a (post) method to create a new lesson based on a class
• [ ] Serializer: For now the class serializer should return title and desc and the list of its lessons
• [ ] Serializer: A lesson should return the id of its class