Fix for massively slow friends timeline query due to indexing bug introduced with repeats.

Sorting on notice.id when our primary selector was notice_inbox.user_id caused a filesort and table scan of the notice table.
Switchng to notice_inbox's notice_id means we can use our index, and everything comes right up.

Before:
mysql> explain SELECT notice.id AS id FROM notice JOIN notice_inbox ON notice.id = notice_inbox.notice_id WHERE notice_inbox.user_id = 18574 AND notice.repeat_of IS NULL ORDER BY notice.id DESC LIMIT 61 OFFSET 0;
+----+-------------+--------------+--------+------------------------------------+---------+---------+-------------------------------+--------+----------------------------------------------+
| id | select_type | table        | type   | possible_keys                      | key     | key_len | ref                           | rows   | Extra                                        |
+----+-------------+--------------+--------+------------------------------------+---------+---------+-------------------------------+--------+----------------------------------------------+
|  1 | SIMPLE      | notice_inbox | ref    | PRIMARY,notice_inbox_notice_id_idx | PRIMARY | 4       | const                         | 102600 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | notice       | eq_ref | PRIMARY                            | PRIMARY | 4       | stoica.notice_inbox.notice_id |      1 | Using index                                  |
+----+-------------+--------------+--------+------------------------------------+---------+---------+-------------------------------+--------+----------------------------------------------+

After:
mysql> explain SELECT notice.id AS id FROM notice JOIN notice_inbox ON notice.id = notice_inbox.notice_id WHERE notice_inbox.user_id = 18574 AND notice.repeat_of IS NULL ORDER BY notice_id DESC LIMIT 61 OFFSET 0;
+----+-------------+--------------+--------+------------------------------------+---------+---------+-------------------------------+--------+--------------------------+
| id | select_type | table        | type   | possible_keys                      | key     | key_len | ref                           | rows   | Extra                    |
+----+-------------+--------------+--------+------------------------------------+---------+---------+-------------------------------+--------+--------------------------+
|  1 | SIMPLE      | notice_inbox | ref    | PRIMARY,notice_inbox_notice_id_idx | PRIMARY | 4       | const                         | 102816 | Using where; Using index |
|  1 | SIMPLE      | notice       | eq_ref | PRIMARY,notice_repeatof_idx        | PRIMARY | 4       | stoica.notice_inbox.notice_id |      1 | Using where              |
+----+-------------+--------------+--------+------------------------------------+---------+---------+-------------------------------+--------+--------------------------+

classes/User.php

@@ -543,7 +543,7 @@ class User extends Memcached_DataObject
 
         // NOTE: we sort by fave time, not by notice time!
 
-        $qry .= 'ORDER BY notice.id DESC ';
+        $qry .= 'ORDER BY notice_id DESC ';
 
         if (!is_null($offset)) {
             $qry .= "LIMIT $limit OFFSET $offset";