Make wgFlowContentFormat = html the default

We aren't running or testing format 'wikitext' and so we have bug 70148
and probably others with it.  It seems better to make the default match
what works, instead of new Flow installations running straight into bugs
we aren't fixing.

Change-Id: I1d5d6bff4d1a24296e42c339436ecf161080fcff
Merge "Make wgFlowContentFormat = html the default"