Create new groups

[Di-az]

User story: Crear nuevos grupos de una materia

• [x] Generate group schema and model in backend
• [x] Generate code of a class once it is created
• [x] Create method to retrieve groups
• [x] Create method to create new group of a class
• [x] Set up endpoints
• [x] Check how to protect endpoints so only teachers can access them

[Di-az]

• [x] Generate code of a class once it is created NOTE: Code wont be generated, but rather provided by the teacher

[Di-az]

Still pending to test it from frontend, should be working