Fix layering so logo is clickable (#8320)

The CSS I added in #8249 to make the search be properly
centred also made the logo unclickable as it was hidden
behind the centred element! This makes the logo stay
above the search container.

ui/app/styles/core/navbar.scss

@@ -41,6 +41,10 @@
       }
     }
 
+    .navbar-brand {
+      z-index: $z-gutter;
+    }
+
     .navbar-end {
       display: flex;
       align-items: stretch;