Add maximum visibility level for forks project setting to extend forking restriction

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To follow-up some discussion in the forking restriction issue we delivered, it would be useful to provide a setting for project admins to define the maximum visibility level for forks. Currently we limit fork visibility as the visibility of the parent, but this could provide more flexibility.

See comments #15111 (comment 214781519) or #15111 (comment 214781525)

cc @jeremy