PYSEC-2026-1692 nbgrader's `frame-ancestors: self` grants all users access to formgrader

Impact Enabling frame-ancestors: 'self' grants any JupyterHub user the ability to extract formgrader content by sending malicious links to users with access to formgrader, at least when using the default JupyterHub configuration of enablesubdomains = False. 1915 disables a protection which would...