Fix redirect after login and do sidebar differently

I noticed that the routes have an option `hidden`. This is what's used to show or hide items in the sidebar.
Now we set this option after login before adding the routes.

The redirect for root is now also set after login before adding the route.
That way we can set the route to the path corresponding with the first item in the sidebar.
When there's no unhidden route (e.g. when admin-fe doesn't know about any privilege the user has), the redirect goes to /401.