UX: Add tooltip text for the timeline notifications button

app/assets/javascripts/discourse/widgets/topic-notifications-button.js.es6

@@ -33,6 +33,10 @@ export default createWidget('topic-notifications-button', {
     if (state.expanded) { return "open"; }
   },
 
+  buildAttributes() {
+    return { title: I18n.t('topic.notifications.title') };
+  },
+
   buttonFor(level) {
     const details = buttonDetails(level);
     return this.attach('button', {

config/locales/client.en.yml

@@ -1310,6 +1310,7 @@ en:
         position: "post %{current} of %{total}"
 
       notifications:
+        title: change how often you get notified about this topic
         reasons:
           "3_6": 'You will receive notifications because you are watching this category.'
           "3_5": 'You will receive notifications because you started watching this topic automatically.'