cells read-only for students should have different appearance

[szazs89]

Operating system

Ubuntu 22.04.4 LTS

nbgrader --version

Python version 3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0]
nbgrader version 0.9.1

jupyterhub --version (if used with JupyterHub)

4.0.2

jupyter notebook --version

7.0.8

Expected behavior

Prevent editing read-only cells for students. It should be clear which cells are to be edited and which are not (e.g. using gray background, or with an explicit text near the cell-frame).

Actual behavior

Now, students can change cell property from Read-only to Editable and edit the cell content.

Steps to reproduce the behavior

Click on the Property Inspector icon in the top right corner, and change the Read-only property.