ImageServing: use content namespace only usage on images when calculating their popularity

https://wikia-inc.atlassian.net/browse/PLATFORM-285

Adding filtering by content namespace articles when counting how many times an image is used on a wiki.

The previous query:

```mysql
explain SELECT 'Potato.jpg' AS il_to FROM `imagelinks` WHERE il_to = 'Potato.jpg' LIMIT 11;
+----+-------------+------------+------+---------------+-------+---------+-------+------+--------------------------+
| id | select_type | table      | type | possible_keys | key   | key_len | ref   | rows | Extra                    |
+----+-------------+------------+------+---------------+-------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | imagelinks | ref  | il_to         | il_to | 257     | const |   39 | Using where; Using index |
+----+-------------+------------+------+---------------+-------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)
```

Query after the change:

```mysql
explain SELECT /* ImageServingDriverMainNS::getImagesPopularity::imagelinks  */ 'Potato.jpg' AS il_to FROM `imagelinks` JOIN page on page.page_id = il_from WHERE il_to = 'Potato.jpg' AND page_namespace IN (0) LIMIT 11 ;
+----+-------------+------------+--------+--------------------+---------+---------+-----------------------------+------+--------------------------+
| id | select_type | table      | type   | possible_keys      | key     | key_len | ref                         | rows | Extra                    |
+----+-------------+------------+--------+--------------------+---------+---------+-----------------------------+------+--------------------------+
|  1 | SIMPLE      | imagelinks | ref    | il_from,il_to      | il_to   | 257     | const                       |   39 | Using where; Using index |
|  1 | SIMPLE      | page       | eq_ref | PRIMARY,name_title | PRIMARY | 4       | plpoznan.imagelinks.il_from |    1 | Using where              |
+----+-------------+------------+--------+--------------------+---------+---------+-----------------------------+------+--------------------------+
2 rows in set (0.00 sec)
```

@Wikia/platform-team / @SQLReview