Add notification system to find replacement teacher if teacher is absent

[renatobellotti]

A possible idea is:

• The teacher notices that he/she cannot give a lesson and activates the notification system on the website.
• People who already finished the course receive an email.
• One of them takes over. If nobody wants to do it before, say, one day before the lesson, the lesson is cancelled and the participants are notified automatically.

Advantage: Once the functionality is implemented and working, no additional administrative effort is needed because everything is automated or handled by the teachers.

[gitsimon]

This functionality would be quit easy once we have a good email sending app inside Django. We can send mass mails already, but we should introduce a nice interface for choosing the receivers from automatically created groups, such as all participants from a course, all woman, all teachers, etc.