Resolve relative links only in namespaces with subpages enabled.

Store which namespaces have subpages enabled in
WikiConfig.namespacesWithSubpages. When a namespace doesn't have
subpages enabled, don't resolve relative links.

Bug: 47136
Change-Id: Ibf7110fb934e2219210abf2351b17d68e55f296e
Merge "Resolve relative links only in namespaces with subpages enabled."