Make modal windows scrollable

Closes twbs#849.

Author: sferik

lib/patterns.less

@@ -834,6 +834,8 @@ input[type=submit].btn {
   top: 50%;
   left: 50%;
   z-index: 11000;
+  max-height: 500px;
+  overflow: auto;
   width: 560px;
   margin: -250px 0 0 -280px;
   background-color: @white;