Show full name and email in “Add Team Member” dropdown picker

<!-- Please only use this template for submitting feature requests --> The team membership dropdown currently shows only usernames. In large orgs or when usernames aren't human-friendly, it's hard to know whom you're adding. [Add a description of the problem the feature is trying to solve.] Display ‘Full Name <email> — username’ in the dropdown. Fall back gracefully if any detail is missing. [Add a brief description of what the feature or enhancement should do.] This feature is for Grafana administrators and team managers in organizations where usernames are not human-friendly or easily recognizable. It especially benefits larger teams and enterprises where multiple users may have similar or auto-generated usernames, and admins need to quickly and accurately identify the correct person to add to a team. [Add information on what kind of user the feature is for.]