Get rid of $wgFlowOccupyPages

That global is being removed in I27e8e45951bfe96453c2bd2424f643a343cb7e59

Meanwhile also start feeding $wgFlowOccupyNamespaces into
$wgNamespaceContentModels: the latter will soon be replacing the former,
in I9d909b1d7d45fd71658b657af09358482981128b

Bug: T105574
Change-Id: I14506483783dce8e59528019a70c4f1cfca145c5