Deleting hidden test cells causes max score to be calculated incorrectly

[meffmadd]

Describe the bug
A user reported that a student had a different max total score from the actual total score of the assignment. They had 35/53 points instead of 35/55. This was because the student deleted a grading cell (by going into the metadata editor and setting it to deletable).

As a consequence, the grading cell did not show up in the grades_dict field of the submission properties. The deleted cell is however registered in the grade_cells_dict field, which is included in the assignment properties and registers which cells are gradable when the assignment is generated. Any discrepancy between these should indicate a deleted cell.

The assignment where the bug was discovered had no task cells so we should check how these affect the behaviour as well.

To Reproduce
Steps to reproduce the behavior:

1. Delete a hidden test cell in an assignment.
2. Submit the assignment.
3. Navigate to the instructor submission view and open the assignment.
4. The total score is incorrect.

Expected behavior
The total score should be correctly calculated, but there should be a warning if grading cells are missing.