Group appointments cannot be cancelled

To reproduce:

1. Create a course with 1 trainer and 3 students
2. Create a group for those 3 students
3. Create a scheduler instance and enable "Booking in groups"
4. Create one future standard time slot (15 min) with no max. number of students
5. Student 1 books the appointment for the whole group -> Result: Button "Cancel booking" is available -> ok
6. Student 1 navigates on the course start page and opens the scheduler again -> Result: Button "Cancel booking" is not available anymore -> should still be available.