Only request background color for stream messages.

subs.get_color is expecting a string, and so if you do this on
a PM, it breaks.

(imported from commit acf6d51b84d7bbbe643466f1f72f90a8f6cf16f6)

zephyr/static/js/zephyr.js

@@ -332,7 +332,9 @@ function add_to_table(messages, table_name, filter_function, where, allow_collap
             message.stamp = ui.get_gravatar_stamp();
         }
 
+        if (message.is_stream) {
             message.background_color = subs.get_color(message.display_recipient);
+        }
 
         messages_to_render.push(message);
         prev = message;