Config setting to disable CI

We need this to make it possible to import pre-8.0 CI data without GitLab downtime. CI should be on by default.

We need to make very clear that a CI import does not merge data; if administrators who intend to import CI data do not turn off CI in 8.0 they risk losing user CI data (builds) created by users between the 8.0 GitLab upgrade and the CI data import.

Part of CI migration improvements in gitlab-org/gitlab-ce#2563.

I take this.

Reassigned to @ayufan

mentioned in issue #2563 (closed)

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1340

Status changed to closed

mentioned in merge request !14785