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.

config/locales/sr.yml

@@ -525,20 +525,6 @@ sr:
     other: Остали
     publishing: Објављивање
     web: Веб
-  push_notifications:
-    favourite:
-      title: "%{name} је ставио Ваш статус за омиљени"
-    follow:
-      title: "%{name} Вас је запратио"
-    group:
-      title: "%{count} обавештења"
-    mention:
-      action_boost: Подржи
-      action_expand: Прикажи још
-      action_favourite: Омиљени
-      title: "%{name} Вас је поменуо"
-    reblog:
-      title: "%{name} је подржао(ла) Ваш статус"
   remote_follow:
     acct: Унесите Ваш корисник@домен са кога желите да пратите
     missing_resource: Не могу да нађем захтевану адресу преусмеравања за Ваш налог