ui: Set overflow: auto on the tab nav (#9402)

This ensures scroll bars are mainly hidden, yet visible on small screens
if the tabs are large enough for there to be an overflow

ui/packages/consul-ui/app/components/tab-nav/layout.scss

@@ -1,6 +1,6 @@
 %tab-nav {
   clear: both;
-  overflow: scroll;
+  overflow: auto;
 }
 %tab-nav ul {
   display: inline-flex;