Support configuration on buckets

[leplatrem]

In our use case for blocklists, it would be better to be able to define the configuration on the staging bucket.

The groups URI could contain placeholder like this:
"/buckets/source/groups/{collection_id}-reviewers"

"hooks": [{
      "event": "kinto_signer.events.ReviewRequested",
      "subject": "{user_id} requested review on {bucket_id}/{collection_id}.",
      "template": "Review changes at {root_url}admin/#/buckets/{bucket_id}/collections/{collection_id}/records",
      "recipients": ["/buckets/source/groups/{collection_id}-reviewers"]
    }]