Allow custom messages on a per-project basis

[rtyley]

Driven by @TBonnin on the Frontend team, let's try to drive a short snippet of infromation to the developer, at the exact moment they need it.

Configuration might look like:

{
    "checkpoints": {
        "PROD": {
            "url": "https://frontend.gutools.co.uk/deploys?stage=PROD&pageSize=50",
            "overdue": "30M"
            "messages": {
              "seen" : "README.md"
              "overdue" : "docs/bad-deploy.md"
            }
        }
    }
}

This would insert the text instead of the default "Please check your changes!" message.

Probably require adding a Map[PullRequestCheckpointStatus,String-Path-Thing] to the CheckpointDetails case class.