Manualgrading does not work, if the lecture is archived

[florian-jaeger]

Describe the bug
Currently, instructors that try to manual grade submissions in a completed lectures can not finish the process.

To Reproduce
Steps to reproduce the behavior:

1. Create a lecture and an assignment
2. Submit the assignment
3. Set the lecture to 'completed'
4. Try manual grading

Expected behavior
The expected behavior is open for discussion:
Either, we throw an error message or we allow manual grading of archived submissions.