Backend: add seeding options

[EwoutV]

Now, it's hard to test with actual data when someone logs in as a test user locally. When DEBUG=True, it would be nice if the following happens when a new user is made:

• Automatically make the user a professor, assistant and student
• Add courses to the student, professor and assitant models, spanning over multiple years
• Add projects to these courses

Next to this, we should look into the package django-faker to seed our database with (a lot of) realistic data, e.g. generate 1000 fake courses with projects, 10.000 fake students...

[EwoutV]

Closed by #248