Thanks to Gomuks' new handling, we have sender displayname and avatar, and can use android's conversation notifications.
On other platforms, we just append sender name to the body, bolded if possible.
This also improves the logic on when to send notifications, as we drop notifs where `response["notification"]` is null.
We also now correctly set if notifications should play a sound or not, based on `PushNotification.sound`.
`PushNotification.image` handling might come sometime in the future, but it seems hard on Android.
