Fix #10979: Don't use .img-thumbnail as a mixin for .thumbnail to avoid dupe and unnecessary styles

dist/css/bootstrap.css

@@ -5198,10 +5198,7 @@ a.list-group-item.active > .badge,
 }
 
 .thumbnail {
-  display: inline-block;
   display: block;
-  height: auto;
-  max-width: 100%;
   padding: 4px;
   margin-bottom: 20px;
   line-height: 1.428571429;

less/thumbnails.less

@@ -5,9 +5,14 @@
 
 // Mixin and adjust the regular image class
 .thumbnail {
-  .img-thumbnail();
-  display: block; // Override the inline-block from `.img-thumbnail`
+  display: block;
+  padding: @thumbnail-padding;
   margin-bottom: @line-height-computed;
+  line-height: @line-height-base;
+  background-color: @thumbnail-bg;
+  border: 1px solid @thumbnail-border;
+  border-radius: @thumbnail-border-radius;
+  .transition(all .2s ease-in-out);
 
   > img {
     .img-responsive();