BAC-483: Special:BrokenRedirects

Always lists interwiki redirects, regardless of validity

Backported a commit from MW 1.21.0 - https://github.com/wikimedia/mediawiki-core/commit/4c21cf53d87697bfbfb521d99b825a3fd287c77e

```mysql
mysql> EXPLAIN SELECT   p1.page_namespace AS namespace,p1.page_title AS title,p1.page_title   AS value,rd_namespace,rd_title  FROM `redirect` JOIN `page` `p1` ON ((rd_from=p1.page_id)) LEFT JOIN `page` `p2` ON ((rd_namespace=p2.page_namespace) AND (rd_title=p2.page_title))  WHERE (rd_namespace >= 0) AND (rd_interwiki IS NULL OR rd_interwiki = '') AND (p2.page_namespace IS NULL)  ORDER BY rd_namespace, rd_title, rd_from LIMIT 50;
+----+-------------+----------+--------+---------------------+-------------+---------+-----------------------------------------------------------+------+--------------------------------------+
| id | select_type | table    | type   | possible_keys       | key         | key_len | ref                                                       | rows | Extra                                |
+----+-------------+----------+--------+---------------------+-------------+---------+-----------------------------------------------------------+------+--------------------------------------+
|  1 | SIMPLE      | redirect | range  | PRIMARY,rd_ns_title | rd_ns_title | 4       | NULL                                                      | 2519 | Using where                          |
|  1 | SIMPLE      | p1       | eq_ref | PRIMARY             | PRIMARY     | 4       | plpoznan.redirect.rd_from                                 |    1 |                                      |
|  1 | SIMPLE      | p2       | eq_ref | name_title          | name_title  | 261     | plpoznan.redirect.rd_namespace,plpoznan.redirect.rd_title |    1 | Using where; Using index; Not exists |
+----+-------------+----------+--------+---------------------+-------------+---------+-----------------------------------------------------------+------+--------------------------------------+
3 rows in set (0.01 sec)
```