Don't require a conversation name

Just display a sensible default in the frontend if it's unset.
For private conversations this should be the phone number, for
groups, the list of numbers.

js/background.js

@@ -141,7 +141,6 @@
                     };
                 } else {
                     attributes = {
-                        name       : source,
                         type       : 'private'
                     };
                 }