(less) Add margin to thumbnails for proper spacing

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

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();