Only check $wgFlowOccupy settings for non-existent pages

Pages that are Flow enabled with content must exist in the page table
and have their content model set to "flow-board".

Setting $wgFlowOccupyNamespaces is nearly equivalent to setting
$wgNamespaceContentModels, except $wgFlowOccupyNamespaces handles
page_content_model=NULL in a sane manner for pages that have not been
touched since $wgContentHandlerUseDB = true.

Since wikitext (and other models) can live in the same occupied
namespace as Flow boards, the restrictions on subpages is removed.

Bug: T95592
Bug: T101881
Bug: T76197
Change-Id: I832c224e36957e39586d151d1902339a57a03399