CONN-619: Maintenance script to add missing comments index

Some old wikis are missing this index ( https://github.com/Wikia/app/blame/dev/extensions/wikia/ArticleComments/patch-create-comments_index.sql#L20 ) from the `comments_index` table. This maintenance script will loop through all wikis, check if the index exists and create it, if it does not. :earth_asia: 

/cc @macbre @nandy-andy can you take a look please
It might not have sense here since it's a maintenance script but there is also [MW solution for creating coma-separated lists](https://github.com/Wikia/app/blob/dev/includes/db/Database.php#L1794) for SQL query.