Use bigger bottom margin in card.html and panel.html (#1323)

Fixes #823

_includes/panel.html

@@ -1,4 +1,4 @@
-<div class="col-xl-4 col-lg-6 col-md-12 mb-2">
+<div class="col-xl-4 col-lg-6 col-md-12 mb-4">
     <div class="card card-{{include.color}}">
         {% if include.color == "warning" %}
         <div class="card-header text-dark bg-{{include.color}}">
@@ -11,4 +11,4 @@
             {{include.body}}
         </div>
     </div>
-</div>
+</div>