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/tr.json

@@ -186,6 +186,7 @@
   "notifications.column_settings.reblog": "Boost’lar:",
   "notifications.column_settings.show": "Bildirimlerde göster",
   "notifications.column_settings.sound": "Ses çal",
+  "notifications.group": "{count} notifications",
   "onboarding.done": "Tamam",
   "onboarding.next": "Sıradaki",
   "onboarding.page_five.public_timelines": "Yerel zaman tüneli, bu sunucudaki herkesten gelen gönderileri gösterir.Federe zaman tüneli, kullanıcıların diğer sunuculardan takip ettiği kişilerin herkese açık gönderilerini gösterir. Bunlar herkese açık zaman tünelleridir ve yeni insanlarla tanışmak  için harika yerlerdir. The federated timeline shows public posts from everyone who people on {domain} follow. These are the Public Timelines, a great way to discover new ",