Message extras field for rendering image in notification

[webworxshop]

As discussed/proposed on #74 add a message extras field containing an image URL and render the notification in BigPictureStyle when this field is present. The proposed message extra structure is as follows:

{
  "extras": {
    "client::notification": {
      "imageUrl": "https://example.com/something.jpg"
    }
  }
}

(source)