CONN-512 Fixing dangling comments_index records left from Special:Nuke

This pull request fixes the root cause of the dangling comments_index records by adding a hook to `ArticleDoDeleteArticleBeforeLogEntry` to mark the comments_index associated with an article as deleted and adds a maintenance script that will mark all of the dangling comments_index records as deleted.

https://wikia-inc.atlassian.net/browse/CONN-512
https://wikia-inc.atlassian.net/browse/CONN-506

/cc @owend