make tooltip work on the scroll button

data/ui.yml

@@ -180,4 +180,5 @@ toggle_images:
   on: Images are on, click to hide
   hide_single: Hide image
   show_single: Show image
-scroll_to_top: Scroll to top
+scroll_button:
+  scroll_to_top: Scroll to top

includes/scroll-button.html

@@ -1,3 +1,3 @@
-<button class="arrow-for-scrolling-top" id="js-scroll-top">
-  <span aria-label="{% data ui.scroll_to_top %}">{% octicon "chevron-up" %}</span>
+<button class="arrow-for-scrolling-top tooltipped tooltipped-n tooltipped-no-delay" aria-label="{% data ui.scroll_button.scroll_to_top %}" id="js-scroll-top">
+  {% octicon "chevron-up" %}
 </button>