Rework Users with ID for Feature Flags

What does this MR do?

Now defines list of feature flags per scope, instead of per flag.

This allows different scopes to enable feature flags for different users!

#34363 (closed)

Screenshots

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers