Previously various language objects would install a hook to update the
shared conversion table cache when the object was constructed. This is
not a good idea since language objects may be constructed even when they
are not the content language, but only the content language is
associated with variant conversion and the conversion cache.
Instead, have WikiPage call a method on $wgContLang directly. I put this
with message cache update since the logic is almost identical.
Change-Id: Ief9c0ef993e39645e74a6e158cb4e6e2139ce91d
