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/ar.yml

@@ -576,19 +576,6 @@ ar:
     other: إعدادات أخرى
     publishing: النشر
     web: الويب
-  push_notifications:
-    favourite:
-      title: أعجب %{name} بمنشورك
-    follow:
-      title: "%{name} من متتبعيك الآن"
-    group:
-      title: "%{count} إخطارات"
-    mention:
-      action_boost: ترقية
-      action_expand: عرض المزيد
-      title: أشار إليك %{name}
-    reblog:
-      title: قام %{name} بترقية منشورك
   remote_follow:
     acct: قم بإدخال عنوان حسابك username@domain الذي من خلاله تود المتابعة
     missing_resource: تعذر العثور على رابط التحويل المطلوب الخاص بحسابك