Add margin to thumbnails for proper spacing

Hadn't thought about this until fixes #10137, but it makes sense I
think to have margin on the bottom of thumbnails for spacing when
stacking (or using different grid tiers).

dist/css/bootstrap.css

@@ -5010,6 +5010,7 @@ a.list-group-item.active > .badge,
   height: auto;
   max-width: 100%;
   padding: 4px;
+  margin-bottom: 20px;
   line-height: 1.428571429;
   background-color: #ffffff;
   border: 1px solid #dddddd;

less/thumbnails.less

@@ -7,6 +7,7 @@
 .thumbnail {
   .img-thumbnail();
   display: block; // Override the inline-block from `.img-thumbnail`
+  margin-bottom: @line-height-computed;
 
   > img {
     .img-responsive();