Add ability to re-grade for all participants

[martinoesterreicher]

It sometimes happens that there is an exercise which was too difficult for the participants and then later needs to be cut out of evaluation.

The possibility to regrade the whole exercise by applying a different individual grade and corresponding maximum grade.

[goggo24]

You can regrade the checkmark grade until no assignments are available. The error message does not make a distinction between these two cases (no assignments and assignments already available). Maybe we could include the core assignment feature "Rescale existing grades" or something similiar to support this. I will put it in our backlog, but due to the covid, we don't have a lot of resources.