Adds a separator above the settings menu

What does this MR do and why?

This adds the separator above the Settings menu (for the new sidebar) under the Project, Group and Admin scopes

How to check and validate locally

Make sure to enable the new sidebar if it is no enabled for you:

1. Enable the feature flag:

echo "Feature.enable(:super_sidebar_nav)" | rails c

2. Enable the user setting from the user dropdown:

After

1. Go to the project page, check that the Settings menu has a separator above it
2. Go to the group page, check that the Settings menu has a separator above it
3. Go to the admin page, check that the Settings menu has a separator above it

Screenshots or screen recordings

Admin	Project	Group

How to set up and validate locally

Related to #410527 (closed)