Refinements to Geo admin page

With https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3902, Geo admin page uses Vue to render node information, but it still needs some updates and refinements as a follow-up for UI polish.

• Update empty state to reflect situation where user has access to Geo feature but does not have any nodes added.
• Styling corrections as mentioned in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3902#note_53684786.
• [ ] Making node Toggle and Remove actions asynchronous (optional, needs gitlab-org/gitlab-ee!3923). Tracked via https://gitlab.com/gitlab-org/gitlab-ee/issues/4658.