Re-enabled nav bar height

IPython/html/static/base/less/variables.less

@@ -1,21 +1,21
 // Our customizations to bootstrap go here.
 
 @black: @gray-darker;
 @text-color: @black;
 @font-size-base: 13px;
 @font-family-monospace: monospace;  // to allow user to customize their fonts
-// @navbar-height: 36px; Not used it BS3
+@navbar-height: 36px;
 
 code {
   color: @black; // default code color in bootstrap is #d14 (crimson / amaranth)
 }
 
 pre {
   // bootstrap has pre defaults that we don't want to inherit.
   // start pre tag defaults based on the surrounding context instead.
   font-size: inherit;
   line-height: inherit;
 }
 
 // Our own global variables for all pages go here