Page skin class fallback

Edge cases arise where we haven't added the `has-page-skin` class to the body, even though a page skin is served

This adds an extra level of safety by checking if a returned ad is a page skin (i.e. it is 1x1 and 'out of page'), and adding the class

cc @gklopper, @kelvin-chappell, @kenlim 
Do we need to check it is a certain type of page here (e.g. is it a front)?