AMBARI-8390. Alerts UI: From alert-definition details page, need breadcrumb to go back to alerts (srimanth)

ambari-web/app/routes/main.js

@@ -340,7 +340,11 @@ module.exports = Em.Route.extend({
       exit: function(router) {
         router.set('mainAlertInstancesController.isUpdating', false);
       }
-    })
+    }),
+
+    back: function (router, event) {
+      window.history.back();
+    }
   }),
 
   admin: Em.Route.extend({

ambari-web/app/templates/main/alerts/definition_details.hbs

@@ -38,7 +38,8 @@
               class="icon-pencil"></i></a>
         {{/unless}}
       </div>
-
+      <div><a href="javascript:void(null)" data-toggle="modal" {{action back}}><i
+            class="icon-arrow-left"></i>&nbsp;{{t common.back}}</a></div>
       <div class="definition-details-block">
         <strong>{{t common.description}}</strong>
         {{#unless controller.editing.description.isEditing}}