Refactor CSP configuration from django-csp to Django 6.0 core support

[thibaudcolas]

This is be a good test of the Django 6.0 capabilities, for @Chiemezuo to work on as part of the Content Security Policy compatibility GSoC project.

Django 6.0 is only due in December, so this won’t be merge-able for a while, but this feels like a good task to do during GSoC nonetheless.