Support Responsive boxShadows in Card component

[hakimelek]

Update boxShadowSize to take an array in addition to a string, for responsive views. We would like to change the shadow in different screen size or remove them in some.

[jes708]

+1, yes this one is important!

[jxnblk]

This should be a fairly straightforward thing to add if we switch to using styled-system's boxShadow function instead of the custom implementation, if anyone wants to open a PR

[bmansell]

Support Responsive boxShadows in Card component

[craigpalermo]

Adding visibility to @bmansell's comment on the trello card

Box shadow in styled-system doesn't seem to support responsive sizes so I'm shelving this one.

[craigpalermo]

We'll want to look into whether or not styled-system supports responsive box shadows to see if that can work alongside our current custom boxShadow function. We may need to change the names of the sizes, which would be a breaking change, so this might need to be a v5 thing.

Styled System: https://styled-system.com/api#shadow

Our implementation: https://github.com/priceline/design-system/blob/master/packages/core/src/utils/boxShadow.js