Add project mirroring controls to namespaces
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=483162) </details> <!--IssueSummary end--> ### 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 <!-- How are you going to track usage of this feature? Think about user behavior and their interaction with the product. What indicates someone is getting value from it? Explore (../../doc/development/internal_analytics/internal_event_instrumentation/quick_start.md) for a guide. --> ### Does this feature require an audit event? Changing this setting should have an audit event
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