Add a CI job that ensure any PR must be validated by at least 2 people

This job will need to be manually triggered, after the MR has received 2 approvals, after which it will be seen as mergeable.

This is the compromise that was found in https://phabricator.wikimedia.org/T368033#10344884 to enable DAGs continuous deployment to the Airflow instances running in Kubernetes, while preventing any single individual to merge malicious code to production and get it deployed.

Signed-off-by: Balthazar Rouberol brouberol@wikimedia.org