Add Ability to Enable Feature Flags by User ID - Frontend

What does this MR do?

Adds the frontend behaviours to allow users to select user IDs to toggle a feature flag for.

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
• Separation of EE specific content

Performance 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

Related to #11459 (closed)