Conn 430 - clean up of varnish purge requests list

**DO NOT MERGE - work in progress**

* new method `ArticleComment::getSquidURLs()`,
* new hook in `ArticleCommentGetSquidURLs`,
* removed logic in Forum which sends the same URLs to the purging list (ArticleComments handles that),
* added Forum namespaces to `GameGuidesController::$disabledNamespaces` array

@macbre @themech @hakubo feel free to comment. I'm going to check tomorrow different actions on Wall/Forum (if they send purge request when they have to) and after that I'll be able to push it to production if you don't find anything nasty in the code ;)