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/gl.yml

@@ -594,20 +594,6 @@ gl:
     other: Outro
     publishing: Publicando
     web: Web
-  push_notifications:
-    favourite:
-      title: "%{name} marcou favorito o seu estado"
-    follow:
-      title: "%{name} agora está a seguila"
-    group:
-      title: "%{count} notificacións"
-    mention:
-      action_boost: Promover
-      action_expand: Mostar máis
-      action_favourite: Favorito
-      title: "%{name} mencionouna"
-    reblog:
-      title: "%{name} promoveu un dos seus estados"
   remote_follow:
     acct: Introduza o seu nomedeusuaria@dominio desde onde quere facer seguimento
     missing_resource: Non se puido atopar o URL de redirecionamento requerido para a súa conta