[Suggestion] fix Skin Remapping functionality

"Skin remapping" allows boards, to assign a specific skin based on expressions in the URL.
This is indeed a very strong feature, which allows for example to map a certain skin to a limited part of the boards.

The current implementation however, allows the admins to map any of the available skins to a certain expression, but this skin is
only shown to the visitors if that skin is also regularly available. If the skin is normally hidden, it will not be used for regular members.

I would like to suggest that even if an skin is hidden, that is is used if it is set by means of "Skin remapping".
This is also the way it works if a skin is assigned to a particular forum.
In my opinion it is more logical and less confusing that way.