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.

Fixes T76772

Bug: T76772
Change-Id: I3fe6be65fbdc4e81c11002e492d821412dd78d2e
Merge "Allow subpage conversion under limited circumstances"