Add missing grade_cells before autograding

[tuncbkose]

Add a new flag OverwriteCells.add_missing_cells, that, when set to True, compares the submitted notebook to the database before autograding and adds missing grade cells to (approximately) correct locations.

Fixes #1682 (with the provided flag).

Notes

• Missing grade cells are added immediately after the previous grade/solution cell, because presumably the notebook structure is repeated blocks of

question students should answer (solution cell)
test for the above (grade cell)
test for the above (grade cell)
...

• The above doesn't apply to manually graded answers, and currently I can't think of a good way to find their correct location based on the database
• Missing cells could be added based on the notebook in the released/ directory, but there is no guarantee they are the same. Instructors can overwrite it, in which case the original notebook might not be recoverable. I am not sure if the database is completely safe, but it should be safer.

To-dos

If this looks good, I will also

• add tests
• extend to restoring task cells as well ("Needs Manual Grade" behaves the same as #1682 if task cells are missing).
• Make the notification configurable?

[github-actions]

👈 Launch a Binder on branch AaltoSciComp/nbgrader/missing_cells

[brichet]

Thanks @tuncbkose.

Fixes #1682 (with the provided flag).

On my side the cell is indeed added, but the status is still need manual grading. I tried deleting an autograded cell.
Is that expected or have I missed something ?

[tuncbkose]

... but the status is still need manual grading.

On a quick attempt, I couldn't reproduce this. I will investigate, but it would be great if you can let me know what you did step by step.

[tuncbkose]

On a more involved attempt, still couldn't reproduce this status mismatch.

[brichet]

On a more involved attempt, still couldn't reproduce this status mismatch.

My fault, it was actually needed a manual grading.
Sorry for the confusion and extra attempts @tuncbkose.

Can you try rebasing on main ?

[tuncbkose]

rebased

[tuncbkose]

This pr shouldn't change anything for ui tests, so failure is unrelated?

[brichet]

Thanks @tuncbkose

[brichet]

@meeseeksdev backport to 0.8.x