need to hook up mailing list subscriptions

[brlcad]

We want to make it so visitors can sign up to our mailing lists with extreme ease, and NOT go through the sourceforge or mailman interface. There's examples for doing this online such as this one: http://szabgab.com/how-to-integrate-a-mailman-signup-with-your-web-site.html

Needs to at least sign them up to brlcad-news, but we'll need to think about how to offer folks sign-up on brlcad-users, brlcad-devel, and/or brlcad-commits without introducing that complication on the main landing page. Perhaps the brlcad-news signup response page can offer them as checkbox options.