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/es.yml

@@ -593,20 +593,6 @@ es:
     other: Otros
     publishing: Publicación
     web: Web
-  push_notifications:
-    favourite:
-      title: "%A {name} le gustó tu estado"
-    follow:
-      title: "%{name} te ha empezado a seguir"
-    group:
-      title: "%{count} notificaciones"
-    mention:
-      action_boost: Retoot
-      action_expand: Mostrar más
-      action_favourite: Me Gusta
-      title: "%{name} te mencionó"
-    reblog:
-      title: "%{name} boosteó tu estado"
   remote_follow:
     acct: Ingesa tu usuario@dominio desde el que quieres seguir
     missing_resource: No se pudo encontrar la URL de redirección requerida para tu cuenta