The RfC Visibility of a consumer is intended to restrict who can access which RfCs. So far, those restrictions were only applied one way, for learners of a restricted consumer to view other (external) RfCs. However, the other way around should also work: If a RfC was created as part of a restricted consumer, no other external user should be able to interfere with this RfC. This commit, therefore, adds this direction as well and covers both directions with tests.
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