Add project mirroring controls to namespaces
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### Problem to solve
On Self-Managed GitLab admins can disable project mirroring; this feature can only be controlled at the instance level which means GitLab.com customers are unable to disable project mirroring.
### Proposal
Allow project mirroring to be disabled at the namespace level for customers on GitLab.com.
### Intended users
* [Sidney (Systems Administrator)](https://handbook.gitlab.com/handbook/product/personas/#sidney-systems-administrator)
* [Amy (Application Security Engineer)](https://handbook.gitlab.com/handbook/product/personas/#amy-application-security-engineer)
* [Alex (Security Operations Engineer)](https://handbook.gitlab.com/handbook/product/personas/#alex-security-operations-engineer)
* [Cameron (Compliance Manager)](https://handbook.gitlab.com/handbook/product/personas/#cameron-compliance-manager)
### Feature Usage Metrics
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### Does this feature require an audit event?
Changing this setting should have an audit event
issue