Switch config from dict to list

[leplatrem]

We may want to send two different emails for the same event. The current approach with the keys do not allow it.

I think also that we could use attributes to filter events, like this:

[
    {
        "resource_name": "record",
        "action": "create",
        "collection_id": "certificates",
        "template": "Hi, a new record '{uri}' has just been created in the collection '{bucket_id}/{collection_id}'",
        "recipients": ['Security reviewers <security-reviews@mozilla.com>'],
        "sender": "Kinto team <developers@kinto-storage.org>"
    },
    {
        "event": "kinto_signer.events.ReviewRequested",
        "template": "Hi, a review is pending your approval on the collection '{bucket_id}/{collection_id}'",
        "recipients": ['Security reviewers <security-reviews@mozilla.com>'],
        "sender": "Kinto team <developers@kinto-storage.org>"
    }
]