Migrate subscribers to GTK mailing lists to Discourse

Now that we have a Discourse instance, we should find a way to tell the people subscribed to gtk-devel-list, gtk-app-devel-list, gtk-list, and gtk-i18n-list that we plan to migrate there.

Sending an email inviting them to create a user on discourse.gnome.org is good, but maybe we should figure out a way to migrate the users themselves? We have their email and password, so it would be pretty easy to get that done.