Creating topic on empty flow page fails

Flow creates a shadow page that is never seen on the wiki on all
flow enabled pages with content, this is needed to convert wikitext
to html specific to that page. It was being created too late in the
process, it is now moved up to header and topic creation so the html
can be generated.

Bug: 58018
Change-Id: Ib8e39618fb2d7df47b94ff14e545a787705d70d6
Merge "Creating topic on empty flow page fails"