Add link to our new Mastodon account.

Include 'rel="me"' so that we can verify this website in Mastodon.

Author: sjrd

_includes/footer.html

@@ -15,6 +15,7 @@ <h4>COMMUNITY</h4>
                 <a class="grey-text text-lighten-3" href="https://discord.com/invite/scala">#scala-js on Discord</a><br/>
                 <a class="grey-text text-lighten-3" href="https://stackoverflow.com/questions/tagged/scala.js">Stackoverflow</a><br/>
                 <a class="grey-text text-lighten-3" href="https://groups.google.com/forum/?fromgroups#!forum/scala-js">Mailing list</a><br/>
+                <a class="grey-text text-lighten-3" rel="me" href="https://fosstodon.org/@scala_js">Mastodon</a><br/>
                 <a class="grey-text text-lighten-3" href="https://twitter.com/scala_js">Twitter <i aria-hidden="true" class="fa fa-twitter"></i></a><br/>
                 <a href="https://github.com/scala-js/scala-js">GitHub <i aria-hidden="true" class="fa fa-github"></i></a>
             </div>