Automate rollback checks when an incident is declared

When an incident is declared via Woodhouse, a production check is initiated and the results are posted to the incident issue.

This production check will now include information on whether or not a rollback of gprd is possible, and a comparison link between the latest deployments.

Original description

We rely on woodhouse to trigger incidents. It generates a slack channel and the incident issue.

It will be a lovely improvement if we could automatically add to the generated incident call the result of the rollback checks /chatops run rollback check gprd