Implement a Danger reminder with checklist to consider validating on pipeline changes

Description

As discussed in the 2020-11-24 EP Team Weekly there should be a reminder when .gitlab/ci/**/* file should remind to consider effects and validate a set of conditions for pipelines:

• Effects on different pipeline types
• Performance issues post merge that warrant communication
• Fork pipeline impacts

To avoid the kind of problems we had with gitlab-com/gl-infra/production#3013 (closed), the template should include a step to communicate all CI changes in Slack.