Add Feature Flag Toggle to Edit Page

What does this MR do?

Part of the frontend for #13416 (closed) which adds a "kill-switch" toggle to the top of a feature flag's details page to allow users to turn the whole flag on or off.

As it is behind the featureFlagToggle flag, I have elected to not add a changelog.

Screenshots

ON

OFF

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• 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