Replace danger-python with Hazard

Hazard is a minimal danger-python replacement.

I tested that the following MR-related issues are being detected with Hazard in place of danger-python: milestone not set; missing MR approval; "Review" label not set; "v9.x" and "Affected v9.x" labels missing; "No CHANGES" label not set when a CHANGES entry is missing; multiple commits present in the MR. It's just a subset of what we have in dangerfile.py, but hopefully enough to demonstrate the replacement works.

A project access token with the Reporter role and api scope needs to be created. The token should be then made available to GitLab CI jobs by setting the DANGER_GITLAB_API_TOKEN CI variable in the BIND 9 project.

Merging should be closely coordinated with isc-projects/images!290.