Merge branch 'layout-nav-zindex' into 'master'

Increased zindex of layout nav

This fixes issues where group inputs would be above the dropdown in the nav

![Screen_Shot_2016-05-17_at_10.14.18](/uploads/08e4a43922051c28377ae6f34725ccc6/Screen_Shot_2016-05-17_at_10.14.18.png)

Closes #17524

See merge request !4176

app/assets/stylesheets/framework/nav.scss

@@ -196,7 +196,7 @@
   position: fixed;
   top: $header-height;
   width: 100%;
-  z-index: 3;
+  z-index: 11;
   background: $background-color;
   border-bottom: 1px solid $border-color;
   transition-duration: .3s;