Skin itself should not be border box

The skin contains various elements which are generated by other
things such as oojs ui and HTML generated from templates written by
editors. Thus limit view-border-box to the things that are in our
control.

Since this patch keeps causing us problems, please at minimum let's have
two reviewers (+1 and +2) before merging.

Bug: T86366
Change-Id: If5fe280e8d4424893cfaee9b1033fa77857287a2