Fix urls under /sky to work again

Turns out that the /sky mapping to the gen
directory was interfering with our new pattern
of mapping the actual /sky directory.

So now we use a more restrictive mapping pattern
of only mapping /sky/services, similar for
/mojo/services and /mojo/public.

Fixed an exception of not finding _sky_server
when a custom sky_server was already running.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/692163002