Fixed sticky validation errors when editing webhooks

refs https://github.com/TryGhost/Ghost/issues/10137
- when closing the edit webhook modal we were rolling back the changed attributes of the webhook model but not clearing the errors object which meant a validation error was shown when re-opening the edit webhook modal
- add `reset()` method to the webhook edit controller and call that when leaving the edit route instead of only rolling back the changed attributes

app/controllers/settings/integration/webhooks/edit.js

@@ -15,5 +15,10 @@ export default Controller.extend({
                 this.transitionToRoute('settings.integration', integration);
             });
         }
+    },
+
+    reset() {
+        this.webhook.rollbackAttributes();
+        this.webhook.errors.clear();
     }
 });

app/routes/settings/integration/webhooks/edit.js

@@ -9,6 +9,6 @@ export default Route.extend({
 
     deactivate() {
         this._super(...arguments);
-        this.controller.webhook.rollbackAttributes();
+        this.controller.reset();
     }
 });