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/sk.json

@@ -186,6 +186,7 @@
   "notifications.column_settings.reblog": "Boosty:",
   "notifications.column_settings.show": "Zobraziť v stĺpci",
   "notifications.column_settings.sound": "Prehrať zvuk",
+  "notifications.group": "{count} notifications",
   "onboarding.done": "Koniec",
   "onboarding.next": "Ďalej",
   "onboarding.page_five.public_timelines": "Lokálna časová os zobrazuje verejné správy od všetkých na {domain}. Federovaná časová os zobrazuje verejné správy od všetkých tých, čo následujú užívatrľov {domain} z iných serverov. Tieto sú takzvané Verejné Časové Osi, výborná možnosť ako nájsť a spoznať nových ľudí.",