Add a way to have a dual grading and dual feedback

[GuillaumeDerval]

A recurring feature request is to be able to set an invisible feedback and an invisible grading together with the feedback and the grading given to the student.

This would allow instructors/teachers/assistants/... to have additionnal informations about specific subtasks, and to hide some information to the students.

[layus]

We had a nice discussion about this after the 1252 exam. Mostly TA want to run grading tests separately from the tests that produce feedback for users.

In may case, I download every submission at the end, and run my grading scripts locally. What prevents others from doing so is that they need to generate the right files based on the submissions and the templates.

Also, it is already possible to get hidden feedback in the output of the run script, accessible with the debug info buttons, visible only by administrators, and downloaded with the submissions.

Batch operations could also be the answer to this need, but they require more visibility/documentation.

-- Just to share ideas.