Customer issues default to private tracker
Created by: pecigonzalo
This PR reinstate the changes proposed in https://github.com/sourcegraph/about/pull/1371 removed in https://github.com/sourcegraph/about/pull/1522.
It changes the default issue tracker for customer issues to be the private customer issue tracker, and only creating an issue in github.com/sourcegraph/sourcegraph
when there is a general problem affecting all or multiple deployments.