Add FF for Registry group browser

What does this MR do?

This adds a feature flag to hide feature (Group container registry browser) till we make it performant, but still allows us to ship it and test ourselves on GitLab.com.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content