Add $wgParsoidWikiPrefix configuration and default to $wgDBname

We defaulted to $wgLanguageCode so far, which does not work so well for
non-Wikipedia projects. The VisualEditor uses $wgDBname instead and configures
the corresponding prefixes in Parsoid's localsettings. Switch to that format
too, so that the VE can actually use our cached content.

Change-Id: I962df0e6e9e5408e6555ec0387a57104c39ceefa
Merge "Add $wgParsoidWikiPrefix configuration and default to $wgDBname"