Update or add multiple users at once. This route is admin-only.

This route is used to update or add multiple users at once. This route reads the provided list of users and updates the user if it already exists, or creates the user if it does not. Any existing users that are not included in the user list will be deleted. The user's username cannot be changed. The user with username "admin" must be excluded from the list of users, as that user can only be self-updated using the "/me" API routes. This route is admin-only.