Allow to disable submissions taking a snapshot of the users home directory

[nbars]

On large homedirs, this takes a long time and also wastes a lot of disk space per submission. Make sure it is noted that this will cause no artifacts except that the submission tests' output and score will be available during grading. In addition to this, we could also consider:

• adding a submission limit that causes retaining only N submissions per exercise per user,
• allow admins to delete the submission instances, e.g., after scoring was done.