add l18n-friendly text for individual image tooltips

Author: sarahs

data/ui.yml

@@ -176,3 +176,5 @@ contributor_callout: This article is contributed and maintained by
 toggle_images:
   hide: Hide images on page
   show: Show images on page
+  hide_single: Hide image
+  show_single: Show image

includes/toggle-images.html

@@ -1,4 +1,6 @@
 <button class="toggle-images tooltipped tooltipped-n" id="js-toggle-images">
   <span id="js-hide-icon" aria-label="{% data ui.toggle_images.hide %}">{% octicon "eye-closed" %}</span>
   <span id="js-show-icon" aria-label="{% data ui.toggle_images.show %}">{% octicon "eye" %}</span>
+  <span id="js-hide-single-image" aria-label="{% data ui.toggle_images.hide_single %}" style="display: none;"></span>
+  <span id="js-show-single-image" aria-label="{% data ui.toggle_images.show_single %}" style="display: none;"></span>
 </button>