`noosphere-sphere` authority writers may only authorize a given DID once

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It should only be possible to authorize a given DID once via SphereAuthorityWrite::Authorize. It's not possible to enforce this as a matter of other implementations, so elsewhere we need to assume there may be multiple authorizations for a given DID. But, from a hygiene perspective, it's best to only have one and revoke/re-issue in certain cases (like when we want to change the authorizer).