Allow custom messages on a per-project basis

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.