UX: Add title text on back button on timeline

app/assets/javascripts/discourse/widgets/topic-timeline.js.es6

@@ -21,6 +21,7 @@ createWidget('timeline-last-read', {
         className: 'btn btn-primary btn-small',
         icon: 'arrow-left',
         label: 'topic.timeline.back',
+        title: 'topic.timeline.back_description',
         action: 'goBack'
       })
     ];

config/locales/client.en.yml

@@ -1295,6 +1295,7 @@ en:
 
       timeline:
         back: "Back"
+        back_description: "Go back to first unread post"
         replies: "%{current} / %{total} replies"
         replies_short: "%{current} / %{total}"