Fix for missing preuploaded presentations

Add a nginx redirect to allow bbb find a pre-uploaded presentation.

ref: https://docs.bigbluebutton.org/greenlight/gl-config.html#updating-from-version-prior-to-27

mod/nginx/bbb/greenlight.nginx

@@ -26,4 +26,9 @@ location /b/cable {
   proxy_send_timeout  6h;
   client_body_timeout 6h;
   send_timeout        6h;
-}
+}
+
+# this is necessary for the preupload_presentation feature
+location /rails/active_storage {
+  return 301 /b$request_uri;
+}