Implement sidebar user menu

What does this MR do and why?

Adds user menu to the super sidebar.

How to set up and validate locally

Prepare

1. Enable the feature flag:

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

2. Make sure to run yarn install as the MR is based on a new feature in gitlab/ui which is in review. That version will be merged before this MR. That's the reason why MR is set to Draft as well as why the Merger Requests menu looks unstyled.
3. Enable the user setting from the user dropdown:

4. The super sidebar should now appear on every page.
5. User menu is in the top right corner of the navigation sidebar
6. Click on it, make sure that all links are navigatable, that you can set/update/ clear status and that it is updated in the dropdown, also that you can perform sign-out.

🐛

Known issues

1. When tabbing to the dropdown toggle (avatar) or pressing Esc in the dropdown the focus is set on the toggle container instead of its content so the focus ring is square instead of rounded. This will be fixed with gitlab-ui#2143 (closed) (Here is the MR gitlab-ui#2143 (closed))
2. The dropdown is positioned to the left of the toggle in the design but apparently, it does not have enough space to fit on the left so it is automatically flipped to the right of the toggle.
3. The menu is missing Buy Pipeline minutes menu item. It is complicated on its own so this will be done in scope of the follow-up.

Related to #385749 (closed)