Add grader preprocessor for cell deletion

[florian-jaeger]

Is your feature request related to a problem? Please describe.
Currently users can theoretically delete cell metadata to delete read only cells in JupyterLab.
Additionally, VSCode, which can also be used to edit notebooks, completely ignores cell metadata and it is therefore easy to delete cells.
This can break the grading of a submission, as the required cell will not be in the notebook.

Describe the solution you'd like
We should add a preprocessor, that detects missing cells and adds them to the notebook during grading.