title: Send message after database update
body: <p>I'm using <code>django-channels</code> to provide a chat app on websockets. I store messages in a database. I would like to be able to send new messages to the client as soon as they get written to the database. How can I do that? I guess I need to make a <code>WebSocketConstructor</code> that will recieve a <code>post_save</code> signal, but I don't know exactly how to send signals to <code>django-channels</code> consumers.</p>