Fixed #15727 -- Added Content Security Policy (CSP) support.

This initial work adds a pair of settings to configure specific CSP
directives for enforcing or reporting policy violations, a new
`django.middleware.csp.ContentSecurityPolicyMiddleware` to apply the
appropriate headers to responses, and a context processor to support CSP
nonces in templates for safely inlining assets.

Relevant documentation has been added for the 6.0 release notes,
security overview, a new how-to page, and a dedicated reference section.

Thanks to the multiple reviewers for their precise and valuable feedback.

Co-authored-by: Natalia <124304+nessita@users.noreply.github.com>

tests/context_processors/urls.py

@@ -5,4 +5,5 @@ from . import views
 urlpatterns = [
     path("request_attrs/", views.request_processor),
     path("debug/", views.debug_processor),
+    path("csp_nonce/", views.csp_nonce_processor),
 ]