fix(messages): sandbox rendered email HTML as extra XSS defence

The app-wide CSP already blocks inline script execution, but the HTML
preview iframe for a stored email was same-origin and un-sandboxed, and
the html_raw response had no per-action hardening. Add a sandbox on the
iframe and tighten the CSP on html_raw to script-src 'none' with
nosniff and no-referrer so the preview has defence in depth against a
future CSP bypass or regression.

Relates to GHSA-f6g9-8555-cw28.

app/controllers/messages_controller.rb

@@ -89,6 +89,18 @@ class MessagesController < ApplicationController
   end
 
   def html_raw
+    override_content_security_policy_directives(
+      default_src: %w('none'),
+      script_src: %w('none'),
+      style_src: %w('unsafe-inline'),
+      img_src: %w(* data:),
+      font_src: %w(*),
+      frame_ancestors: %w('self'),
+      form_action: %w('none'),
+      base_uri: %w('none')
+    )
+    response.headers["X-Content-Type-Options"] = "nosniff"
+    response.headers["Referrer-Policy"] = "no-referrer"
     render html: @message.html_body_without_tracking_image.html_safe
   end