Add the same border to images as used in GitLab

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4655

stylesheets/styles.css

@@ -101,6 +101,9 @@ th {
 img {
   max-width: 100%;
+  border: 1px solid rgb(240, 240, 240);
+  padding: 5px;
+  margin: 5px;
 }
 header {