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/actions/notifications.js

@@ -22,6 +22,7 @@ export const NOTIFICATIONS_SCROLL_TOP = 'NOTIFICATIONS_SCROLL_TOP';
 
 defineMessages({
   mention: { id: 'notification.mention', defaultMessage: '{name} mentioned you' },
+  group: { id: 'notifications.group', defaultMessage: '{count} notifications' },
 });
 
 const fetchRelatedRelationships = (dispatch, notifications) => {