Relocate admin mode switch from command palette to user menu

What does this MR do and why?

Relocate admin mode button to user menu (#431434 - closed)

Screenshots or screen recordings

enter	leave

How to set up and validate locally

• The Admin Mode feature first hast to be enabled on your instance as described in the docs.
• Now, as an admin, you should find the "Enter/Leave Admin Mode" links in the user dropdown, instead of the command/search modal.
• Enter Admin Mode and leave it again to make sure the links still work.
• Impersonate a non-admin user and make sure none of the links show up!
• Disable the Admin Mode feature again and make sure none of the links show up, neither for admins nor non-admins.