Add starred messages to our new API for unread ids.

Even though starred messages are never unread, it's useful
for us to have helper functions for them.

This change makes it so that clicking on "Starred Messages"
takes you to the last read message immediately, without a
server delay.

static/js/unread.js

@@ -539,6 +539,14 @@ exports.get_msg_ids_for_mentions = function () {
     return util.sorted_ids(ids);
 };
 
+exports.get_msg_ids_for_starred = function () {
+    // This is here for API consistency sake--we never
+    // have unread starred messages.  (Some day we may ironically
+    // want to make starring the same as mark-as-unread, but
+    // for now starring === reading.)
+    return [];
+};
+
 exports.load_server_counts = function () {
     var unread_msgs = page_params.unread_msgs;