Remove top and bottom margin for hidden checklist items

Remove top and bottom margin for hidden checklist items, otherwise there could
be a gap between unchecked items if multiple hidden/checked items were between
them.

client/components/cards/checklists.styl

@@ -112,6 +112,8 @@ textarea.js-add-checklist-item, textarea.js-edit-checklist-item
     opacity: 0
     height: 0
     transition: height 0ms 0ms, opacity 600ms 0ms
+    margin-top: 0
+    margin-bottom: 0
 
   &.placeholder
     background: darken(white, 20%)