add wgARENamespaces to ManageWikiNamespaces (#3661)
- add wgARENamespaces to ManageWikiNamespaces
See https://meta.miraheze.org/w/index.php?title=Community_noticeboard&oldid=156690#Problem_with_ArticleRatings_extension for extra details
- wgARENamespaces default to $wgContentNamespaces
the overridedefault option seems to require an associative array, and $wgContentNamespaces isn't one, so let's convert it!
- ensure default is false