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/test_navigation.py

@@ -212,6 +212,7 @@ EXCLUDED_ENDPOINTS = tuple(map(Navigation.get_endpoint_with_blueprint, {
     'roadmap',
     'save_contact_list',
     'security',
+    'security_policy',
     'send_files_by_email',
     'send_files_by_email_contact_details',
     'send_from_contact_list',