export subtree to tar file

src/views/index.ejs

@@ -497,6 +497,7 @@
     <script src="javascripts/drag_and_drop.js"></script>
     <script src="javascripts/context_menu.js"></script>
     <script src="javascripts/search_tree.js"></script>
+    <script src="javascripts/export.js"></script>
 
     <!-- Note detail -->
     <script src="javascripts/note_editor.js"></script>