Add security policy page

This follows the guidance in
https://gds-way.cloudapps.digital/standards/vulnerability-disclosure.html#vulnerability-disclosure-and-security-txt

tests/app/main/views/test_security_policy.py

@@ -0,0 +1,13 @@
+import pytest
+
+
+@pytest.mark.parametrize('url', [
+    '/security.txt',
+    '/.well-known/security.txt',
+])
+def test_security_policy_redirects_to_policy(client_request, url):
+    client_request.get_url(
+        url,
+        _expected_status=302,
+        _expected_redirect="https://vdp.cabinetoffice.gov.uk/.well-known/security.txt",
+    )