Add a seat tasks config for Workshop experience.

Only the actual workshop gives Workshop experience. This seems reasonable
as it is a dedicated part.
Use KerbalStats for workshop experience.

If KerbalStats is not installed, then everything should be as before, but
if KerbalStats is installed, then given enough time, *all* kerbals will
become productive. However, stupid kerbals will improve more slowly than
smart kerbals.