Re-activate Notebook validation

[turnmanh]

In order to push notebooks with a commented load stmt. - s.t. participants don't accidentally see the solution - the validation was deactivated in a589d85. However, it is a useful check to not hand out notebooks with solutions included. Therefore, it has to be re-activated with a nice workaround s.t. the %load magic command not executed by accident.

      - name: validate notebooks
        run: |
          for notebook in notebooks/*.ipynb; do
            if grep -q '# *%load ' "$notebook"; then
              echo "$notebook contains an executed load statement. 
          This is not allowed inside the docker image for participants, make sure you remove this
          in a future commit."
              exit 255
            fi
          done