fix(GlListbox): move loading icon to list container

mentioned in commit 26046af2

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!116 (merged)

mentioned in merge request status-page!670 (merged)

mentioned in merge request status-page!671 (merged)

mentioned in merge request gitlab!108150 (merged)

mentioned in merge request gitlab-docs!3416 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3204 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!117 (merged)