Simplify workflow configuration

[leplatrem]

At the origins of kinto-signer, we had no preview collection.

We only had source -> destination and the to_review_enabled setting.

We introduce preview afterwards, and had two notions: review enabled and presence of preview collection.

In Remote Settings, we could simplify that and keep only have one notion:

• all collections would have a preview collection (all would fall under /buckets/X -> /buckets/Y -> /buckets/Z
• specific collections would have to_review_enabled = False

We could also remove the ability to have different collections names for preview and destination. For example, drop support of: /buckets/X/collections/A -> /buckets/X/collections/B -> /buckets/X/collections/C.

Caution: we may have to maintain some metadata in the capabilities, since it's used by Kinto Admin. See https://github.com/Kinto/kinto-admin/blob/e5792ea176192d42d61086588f9622e40876ee9b/src/plugins/signoff/sagas.ts#L378-L394

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Imported from mozilla-services/kinto-dist#1946