jquery-ui: fix autocomplete menu styling (#3671).

* Wrap menu items in a div.
* Replace ui-state-focus with ui-state-active.
* Make active items have a grey background.

ref: http://jqueryui.com/upgrade-guide/1.12/#require-wrappers-for-each-menu-item
ref: http://jqueryui.com/upgrade-guide/1.12/#use-consistent-styling-for-focused-and-active-items

app/assets/stylesheets/common/jquery_ui_custom.scss

@@ -57,3 +57,7 @@ div.ui-dialog {
     background: linear-gradient(#fefefe, #f0f0f0);
   }
 }
+
+.ui-front {
+  z-index: 1000;
+}