Add muting_enabled option to MessageList.

(imported from commit c86772b7dea864c49091c988772e17aae5efa5bc)

static/js/message_list.js

@@ -2,6 +2,7 @@
 function MessageList(table_name, filter, opts) {
     _.extend(this, {
         collapse_messages: true,
+        muting_enabled: true,
         summarize_read: false
     }, opts);
     this.view = new MessageListView(this, table_name, this.collapse_messages);