Move API group deletion to Sidekiq
Many SQL queries are necessary to delete a group, and this can trigger the QueryLimiter limits. With enough projects, this deletion can time out after 60 seconds.
This is necessary to fix QueryLimiter failures in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18609.
EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5505
Edited by Stan Hu