Add task for checking plausibility of data

• Celery background task
• Email notification
• Change icon in menu
• Permissions
• Doc strings
• Custom health check (https://github.com/KristianOellegaard/django-health-check#writing-a-custom-health-check)
• Class-based views
• Simplify result registration

Tasks to implement

• Personal notes for cancelled lessons
• Groups of person must be set in personal notes
• Lesson documentation entries
• Personal notes on holidays
• Excused personal notes without absence
• Solve option: clear