add card column margin variable instead of relying on a default card margin

this assumes no margin on the .card to start (which will follow in a later commit)

scss/_card.scss

@@ -286,6 +286,7 @@
     .card {
       display: inline-block; // Don't let them vertically span multiple columns
       width: 100%; // Don't let them exceed the column width
+      margin-bottom: $card-columns-margin;
     }
   }
 }

scss/_variables.scss

@@ -709,6 +709,7 @@ $card-deck-margin:         .625rem !default;
 
 $card-columns-count:        3 !default;
 $card-columns-gap:          1.25rem !default;
+$card-columns-margin:       $card-spacer-y !default;
 
 
 // 22. Tooltips