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/views/messages/html.html.haml

@@ -14,4 +14,4 @@
         This means that we no longer store the raw data for this e-mail
         or the e-mail didn't include a HTML part.
 - else
-  %iframe{:width => "100%", :height => "100%", :src => html_raw_organization_server_message_path(organization, @server, @message.id)}
+  %iframe{:width => "100%", :height => "100%", :sandbox => "allow-popups allow-popups-to-escape-sandbox", :referrerpolicy => "no-referrer", :src => html_raw_organization_server_message_path(organization, @server, @message.id)}