Remove default event handler from theme switch links

On Chrome the default <a> event handler was firing after reload()
resulting in a change of location.

This was particularly noticeable on the new angular Swift UI.

Change-Id: I5b644f3b899a5a8c598ceae7cdb76a1e9fd89ebf
Closes-Bug: 1555429

openstack_dashboard/templates/header/_theme_list.html

@@ -27,6 +27,10 @@
       $(document).on('click', '.theme-picker-item', function(e) {
         var $this = $(this);
 
+        // prevent the default <a> click handler from firing - on
+        // Chrome it confuses the reload() we do below
+        e.preventDefault();
+
         if($this.hasClass('disabled')) {
           e.stopPropagation();
           return;