Add a more permissive access right

The most permissive right is "view", but some actions need an even more permissive right. For example, if the wiki requires authentication for viewing, then the skin will not be displayed.

In the case of the skin, we could put skin documents in a separate space, and require that space to have public view rights, but this is not nice/extensible. Instead, we should add a new access right class, like "public", which cannot be used in the Access Rights Editor, but is used internally to allow some actions to always be executed, regardless of the access rights of the user.

I think that the skin is not something that should not be publicly available, so it would be OK to just allow /skin/ requests to be executed.