Include path in web push payload for backwards compatibility

Old service workers cached in browsers still read `data.path` from the
push notification payload. Include it alongside `data.url` so notification
clicks keep working until those service workers update.

Currently we only register the service worker in the notification
settings screen, so this won't happen very often. Once we introduce
offline mode, we'll register the service worker in more places, so we
should be able to remove this.

lib/web_push/notification.rb

@@ -20,7 +20,7 @@ class WebPush::Notification
     end
 
     def encoded_message
-      JSON.generate title: @title, options: { body: @body, icon: icon_path, data: { url: @url, badge: @badge } }
+      JSON.generate title: @title, options: { body: @body, icon: icon_path, data: { url: @url, path: @url, badge: @badge } }
     end
 
     def icon_path