fix admin default page

When Kooboo is not in the root directory of the domain, admin is not redirected to the correct site.

Example: Kooboo is installed on somesite.com/kooboo. After login admin is redirected to somesite.com/Sites/Home/Cluster when instead he should be redirected to somesite.com/kooboo/Sites/Home/Cluster

This behavior is fixed with this change.