Read-only view of assignments for student tutors

[troeger]

This is currently impossible with Django:

https://code.djangoproject.com/ticket/8936

This leads to the fact that student tutors get a 404 whenever they want to investigate assignment details in the teacher backend. This relates to both the list of assignments and single assignment details.

If Django ever adds this feature, then we can make the permission part of the student tutors permission set. Until then, single installations can assign the "change permission" for assignments to the group on an individual base.

A programmatic fix might be to have an own read-only rendering of assignment details.