Improve payload format of Web Push API now that it's open (#7521)

> Good lord what is happening in there

Previously the contents of the Web Push API payloads closely resembled the structure of JavaScript's [Notification](https://developer.mozilla.org/en-US/docs/Web/API/Notification). But now that the API is open to non-browser apps, and given that there is no required coupling between contents of the payload and a Notification object, here is how I changed the payload:

```json
{ 
  "access_token": "...",
  "preferred_locale": "en",
  "notification_id": "12345",
  "notification_type": "follow",
  "title": "So and so followed you",
  "body": "This is my bio",
  "icon": "https://example.com/avatar.png"
}
```

The title, body and icon attributes are included as a fallback so you can construct a minimal notification if you cannot perform a network request to the API to get more data.

app/javascript/mastodon/locales/zh-CN.json

@@ -186,6 +186,7 @@
   "notifications.column_settings.reblog": "当有人转嘟了你的嘟文时：",
   "notifications.column_settings.show": "在通知栏显示",
   "notifications.column_settings.sound": "播放音效",
+  "notifications.group": "{count} notifications",
   "onboarding.done": "出发！",
   "onboarding.next": "下一步",
   "onboarding.page_five.public_timelines": "“本站时间轴”显示的是由本站（{domain}）用户发布的所有公开嘟文。“跨站公共时间轴”显示的的是由本站用户关注对象所发布的所有公开嘟文。这些就是寻人好去处的公共时间轴啦。",