Add a social pointer to Mastodon (specifically, our new official home at Fosstodon). This also serves as a verification link for Fosstodon, and the rel="me" is required, which is why it's being handled out-of-band from the other links; if we're comfortable always using rel="me" for social links, we could unify this.

Fixes #403.

Author: Mark "Justin du Coeur" Waks

_includes/_footer.html

@@ -31,6 +31,7 @@ <h3>Scala</h3>
                 </a>
                 </li>
                 {% endfor %}
+                <li><a rel="me" href="https://fosstodon.org/@typelevel" target="_blank"><i class="fab fa-mastodon"></i></a></li>
             </ul>
         </div>
     </div>