It makes more sense to add the module via the site directory

In addition to the global gluon modules file, there can be a site
specific modules file.

Using that, it's not necessary to manually edit the gluon modules file
to add the offline-ssid feature.

see: https://github.com/ffac/site/commit/16f620e4bb3a1cc27d50934a22e8281278145397