Add Matomo tracking code

URL and site id are hardcoded, but I don't think it matters too much
considering that repository is Totem-specific anyway?

source/javascripts/livestats.js

+// Load the Matomo script to gather anonymised statistics:
+/* global Piwik */
+$(function() {
+  var piwikTracker = Piwik.getTracker('https://livestats.totem-project.org/matomo.php', 8);
+  piwikTracker.trackPageView();
+  piwikTracker.enableLinkTracking();
+});

source/layouts/layout.erb

@@ -5,7 +5,7 @@
     <meta http-equiv="x-ua-compatible" content="ie=edge">
     <meta http-equiv="x-ua-compatible" content="ie=edge">
     <meta http-equiv="X-XSS-Protection" content="1; mode=block">
-    <meta http-equiv="Content-Security-Policy" content="default-src 'self'; font-src 'self' fonts.gstatic.com; style-src 'self' fonts.googleapis.com; img-src 'self' learn.totem-project.org;">
+    <meta http-equiv="Content-Security-Policy" content="default-src 'self'; font-src 'self' fonts.gstatic.com; style-src 'self' fonts.googleapis.com; img-src 'self' learn.totem-project.org; script-src 'self' https://livestats.totem-project.org; connect-src 'self' https://livestats.totem-project.org">
     <meta name="viewport"
           content="width=device-width, initial-scale=1, shrink-to-fit=no">
     <meta name="description" content="Join the totem project to learn more about security on the web">
@@ -46,4 +46,6 @@
     <%= yield %>
     <%= partial "partials/footer" %>
   </body>
+  <%= javascript_include_tag "https://livestats.totem-project.org/piwik.js" %>
+  <%= javascript_include_tag "livestats" %>
 </html>