Add link to Google Groups mailing list in footer's social section

[geerlingguy]

Alongside the GitHub and Twitter links, we should add a link to the Google Groups mailing list: https://groups.google.com/forum/#!forum/open-source-catholic

We'll need to add some sort of mail icon. In fact, as part of this issue, I might also think about taking out the two existing SVG icons and switching to using FontAwesome, loaded via CDN or locally, instead, since that would allow us a lot more flexibility icon-wise!

[geerlingguy]

Still want to do this. I keep going to the site to get to Google Groups for reference, and I forget that there's no link to the Google Groups!