Update to use setSkin and ignore capitalisation

Internal skin names will always be lowercase, so to simplify things
for the users we might as well automatically lowercase the name
here.

$wgUser->mSkin was deprecated at some point, but now we can just
directly set the skin.

Change-Id: If73797c55c2bb9e61367c3eb27757e12f3285503
(cherry picked from commit cb4978231d7c9e6f7ed6a41da4956eb6ab9323ed)