if there are no external services, redirect to external-services-add

Created by: beyang

During a recent user onboarding, @slimsag and I witnessed user confusion that occurred when a new user wanted to add repositories to Sourcegraph, navigated to the external services page, but didn't understand what to do (click the blue button to "add a new external service"). We should just redirect to the "add external service" page if there are no existing external services.

should redirect to