Move social links to the footer

[sethaxen]

We should link to our Mastodon account from the Turing page (specifically, using <a rel="me" href="https://bayes.club/@TuringLang">) so we can authenticate our Mastodon account. But there's not quite enough room on the top bar, and in fact, that top bar is already too crowded (often for me the search bar is cut off on the right before the top bar turns into a hamburger menu.

I propose we move the social links, including Mastodon, to the website footer.