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/check_framework/test_security.py

@@ -1,3 +1,5 @@
+import itertools
+
 from django.conf import settings
 from django.core.checks.messages import Error, Warning
 from django.core.checks.security import base, csrf, sessions
@@ -678,3 +680,54 @@ class CheckCrossOriginOpenerPolicyTest(SimpleTestCase):
     )
     def test_with_invalid_coop(self):
         self.assertEqual(base.check_cross_origin_opener_policy(None), [base.E024])
+
+
+class CheckSecureCSPTests(SimpleTestCase):
+    """Tests for the CSP settings check function."""
+
+    def test_secure_csp_allowed_values(self):
+        """Check should pass when both CSP settings are None or dicts."""
+        allowed_values = (None, {}, {"key": "value"})
+        combinations = itertools.product(allowed_values, repeat=2)
+        for csp_value, csp_report_only_value in combinations:
+            with (
+                self.subTest(
+                    csp_value=csp_value, csp_report_only_value=csp_report_only_value
+                ),
+                self.settings(
+                    SECURE_CSP=csp_value, SECURE_CSP_REPORT_ONLY=csp_report_only_value
+                ),
+            ):
+                errors = base.check_csp_settings(None)
+                self.assertEqual(errors, [])
+
+    def test_secure_csp_invalid_values(self):
+        """Check should fail when either CSP setting is not a dict."""
+        for value in (
+            False,
+            True,
+            0,
+            42,
+            "",
+            "not-a-dict",
+            set(),
+            {"a", "b"},
+            [],
+            [1, 2, 3, 4],
+        ):
+            with self.subTest(value=value):
+                csp_error = Error(
+                    base.E026.msg % ("SECURE_CSP", value), id=base.E026.id
+                )
+                with self.settings(SECURE_CSP=value):
+                    errors = base.check_csp_settings(None)
+                    self.assertEqual(errors, [csp_error])
+                csp_report_only_error = Error(
+                    base.E026.msg % ("SECURE_CSP_REPORT_ONLY", value), id=base.E026.id
+                )
+                with self.settings(SECURE_CSP_REPORT_ONLY=value):
+                    errors = base.check_csp_settings(None)
+                    self.assertEqual(errors, [csp_report_only_error])
+                with self.settings(SECURE_CSP=value, SECURE_CSP_REPORT_ONLY=value):
+                    errors = base.check_csp_settings(None)
+                    self.assertEqual(errors, [csp_error, csp_report_only_error])