add opt out for mixpanel tracking

app/views/layouts/default.html.erb

@@ -84,6 +84,14 @@
       $(window).trigger("danbooru:error", msg);
     }
   </script>
+  <script>
+    window.Danbooru.mixpanelInit(
+      <%= CurrentUser.id %>,
+      <%= raw session.id.to_json %>,
+      <%= CurrentUser.opt_out_mixpanel? %>,
+      <%= raw({"$created" =>CurrentUser.created_at.strftime('%F %T'), "$name" => CurrentUser.name, "level" => CurrentUser.level, "upload_count" => CurrentUser.post_upload_count, "post_update_count" => CurrentUser.post_update_count, "note_update_count" => CurrentUser.note_update_count, "favorite_count" => CurrentUser.favorite_count}.to_json) %>
+    );
+  </script>
 
 </head>
 <%= tag.body **body_attributes(CurrentUser.user) do %>