Allow subpage conversion under limited circumstances
Adjusts the pre-checks done on the converter so that we
allow conversion of subpages if and only if they are
within a talk namespace and have a matching subject page.
Note that this will not work in combination with
$wgFlowOccupyNamespaces, as that explicitly does not take
over sub pages. It will work fine on wikis with
$wgContentHandlerUseDb set to true.