Don't store conversationType on messages

This was used to conditionally render messages in the group style, but
it's actually unnecessary. We can render the same markup in both cases
and change the appearance with css.

js/views/message_view.js

@@ -61,7 +61,7 @@
             message: this.model.get('body'),
             timestamp: moment(this.model.get('timestamp')).fromNow(),
             bubble_class: this.model.get('type') === 'outgoing' ? 'sent' : 'incoming',
-            sender: this.model.get('conversationType') === 'group' ? this.model.get('sender') : ''
+            sender: this.model.get('sender')
           })
         );