Implement User menu in super sidebar navigation

Proposal

Using <GlDisclosure>, implement the user menu in the super sidebar navigation. Clicking the avatar triggers the disclosure, clicking outside closes it. It's important we do this early on as it contains the means to toggle the new navigation back off.

Things to note

• Toggle component exists in app/assets/javascripts/nav/components/new_nav_toggle.vue
• "Provide feedback" will open GITLAB_ISSUE in a new window
• This will be behind both the super_sidebar_nav feature flag and the navbar toggle.

Designs

See attached design files and the Navigation redesign file in figma.

Please ensure the MR is reviewed by UX