EE: Rewrite the GitHub importer to perform work in parallel and greatly improve performance
This is the EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14731 with all merge conflicts resolved, based on EE master.
This is the EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14731 with all merge conflicts resolved, based on EE master.