Fix ArticlePurge hook

* Actually register the hook
* Hook will provide a WikiPage object, not an article
* Introduce HelpPages::purgeCache which actually clears the cache

Change-Id: If109114eb0ed1efa543452be09bea36f8501af43
Set visibility on some stuff

Change-Id: I708cb635022471ba057f0b5f735c397152fa37fb