Add support for admin mode to the new navigation

What does this MR do and why?

The admin mode feature is toggled on/off via a button in the old nav. We are now adding a toggle to the new navigation.

#406757 (closed)

Screenshots or screen recordings

How to set up and validate locally

To enable this feature locally, you need to run the following commands in the rails console:

Feature.enable(:super_sidebar_nav)

Feature.enable(:user_mode_in_session)
::Gitlab::CurrentSettings.update!(admin_mode: true)

This will enable the new navigation and admin mode.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.