Default $wgVisualEditorNamespaces to $wgContentNamespaces

The domains which contains articles are those from $wgContentNamespaces,
which by default is just array( NS_MAIN ).

Bug: 51527
Change-Id: I1f1356f1950c542ae05095a9ebd7b3d66ceb4e40
Merge "Default $wgVisualEditorNamespaces to $wgContentNamespaces"