Enlarge and move Jenkins head icon

Enlarge and move Jenkins head image a bit to be flush with the bottom of
the header bar.

war/src/main/webapp/css/style.css

@@ -52,7 +52,7 @@ body {
 
 #jenkins-head-icon {
   position: relative;
-  top: 2px;
+  top: 4px;
 }
 #jenkins-name-icon {
   position: relative;