feat: Update loading icon spinner

Remove the dependancy on font awesome and make use of css from gitlab-ce application.scss instead.

Update tests and documentation

Needed for https://gitlab.com/gitlab-org/gitlab-ce/issues/20084
Bug fix after testing: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25683

MR where application.scss is updated: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25186

Follow up issue created for number to string refactoring.