Add support for admin mode to the new navigation

The admin mode feature is toggled on/off via a button in the old nav. The new nav doesn't have this feature so may need to be added.

Proposal

Update app/helpers/sidebars_helper.rb to include the link in the context switcher. You can see the logic for the current implementation here: /app/helpers/nav/top_nav_helper.rb#L202

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

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

Docs link for reference: https://docs.gitlab.com/ee/user/admin_area/settings/sign_in_restrictions.html#turn-on-admin-mode-for-your-session