VID-492 Ignore known premium video in the WantedFiles page

@SQLReview 

The code adds to an existing SQL query. The resulting query is:

```sql
SELECT
    '6' AS namespace,
    il_to AS title,
   COUNT(*) AS value
FROM `imagelinks`
    LEFT JOIN `image`
        ON il_to = img_name
    LEFT JOIN `video_info`
        ON il_to = video_title AND premium = 1
WHERE
    img_name IS NULL AND
    il_to not in ('Placeholder','Welcome-user-page') AND
    video_title IS NULL
GROUP BY il_to
ORDER BY value DESC
LIMIT 1000
```

The parts added by this ticket are:
```sql
    LEFT JOIN `video_info`
        ON il_to = video_title AND premium = 1
    # ...
    video_title IS NULL
```

The explain for this query is:
```
| id | select_type | table      | type   | possible_keys   | key     | key_len | ref                    | rows | Extra                                                     |
|  1 | SIMPLE      | imagelinks | index  | il_to           | il_to   | 261     | NULL                   |  178 | Using where; Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | image      | eq_ref | PRIMARY         | PRIMARY | 257     | garth.imagelinks.il_to |    1 | Using where; Using index; Not exists                      |
|  1 | SIMPLE      | video_info | eq_ref | PRIMARY,premium | PRIMARY | 257     | garth.imagelinks.il_to |    1 | Using where; Not exists                                   |
```