Add Mastodon to Jupyter web site (#710)

Author: JasonWeill

_layouts/default.html

@@ -55,6 +55,10 @@
     <link rel="stylesheet" href="/assets/css/bootstrap.css">
     <link rel="stylesheet" href="/assets/css/main.css">
     <link rel="icon" type="image/x-icon" href="/favicon.ico">
+    <!-- These 'rel="me"' links validate that the Fosstodon Mastodon accounts below are officially associated with Project Jupyter -->
+    <!-- The @Jupyter account is primary and @ProjectJupyter is a backup -->
+    <link rel="me" href="https://fosstodon.org/@Jupyter">
+    <link rel="me" href="https://fosstodon.org/@ProjectJupyter">
     <link rel="apple-touch-icon" href="/assets/favicons/apple-touch-icon-152x152.png">
     <link rel="apple-touch-icon" sizes="120x120" href="/assets/favicons/apple-touch-icon-120x120.png">
     <link rel="apple-touch-icon" sizes="152x152" href="/assets/favicons/apple-touch-icon-152x152.png">
@@ -159,4 +163,4 @@ <h2>Follow us</h2>
     </footer>
     <script src="/assets/js/bootstrap-native.min.js" defer></script>
   </body>
-</html>
+</html>