Migrate to Django SECURE_REFERRER_POLICY

[thibaudcolas]

The site currently uses the django-referrer-policy package, however this should no longer be needed.

Current implementation:

# Referrer-policy header settings
# https://django-referrer-policy.readthedocs.io/en/1.0/

REFERRER_POLICY = env.get(
    "SECURE_REFERRER_POLICY", "no-referrer-when-downgrade"
).strip()

Once we migrate to the built-in support:

# https://docs.djangoproject.com/en/stable/ref/middleware/#referrer-policy
SECURE_REFERRER_POLICY = env.get(
    "SECURE_REFERRER_POLICY", "no-referrer-when-downgrade"
).strip()

[p-r-a-v-i-n]

Hey @thibaudcolas ,
I'd like to take this up!

As per your note, I’ll:

Remove the django-referrer-policy package.

Replace REFERRER_POLICY with Django’s built-in SECURE_REFERRER_POLICY in settings.py.

Remove any middleware references, if present.

Please let me know if there’s anything else you'd like me to cover.
happy to include tests or docs if needed. Otherwise, I’ll go ahead and open a PR soon 😊