Forks should inherit permissions from upstream project
Private projects can be forked by individuals causing the original project maintainer to lose access and control. This can also prevent merge requests being opened. We should give maintainers of private projects access to forks.
This would make it possible for the Allow edits by maintainers option to be enabled for Private projects.
This is also consistent with GitHub's forking model (https://help.github.com/articles/about-forks/):
Private forks inherit the permissions structure of the upstream or parent repository. For example, if the upstream repository is private and gives read/write access to a team, then the same team will have read/write access to any forks of the private upstream repository. This helps owners of private repositories maintain control over their code.
Maintainers of private projects should have maintainer permissions over forks to retain access and control