add "play" icon for "render" note types

src/public/javascripts/note_tree.js

@@ -157,6 +157,9 @@ const noteTree = (function() {
         if (note.type === 'code') {
             extraClasses.push("code");
         }
+        else if (note.type === 'render') {
+            extraClasses.push('render');
+        }
         else if (note.type === 'file') {
             extraClasses.push('attachment');
         }