Make $card-deck-margin default to $grid-gutter-width-base / 2

Created by: aRestless

Cards are a cool concept, but with default values card-decks are somewhat incompatible to mix-and-match with elements designed with the grid system, see https://jsbin.com/butapaqucu/edit?html,css,output, which results in a layout with irregular (i.e. bad) spacing.

Setting $card-deck-margin to $grid-gutter-width-base / 2 by default would result in the same spacing for all these elements, regardless of origin.