chore: Migrate GlLoadingIcon styles to gitlab-ui

What does this MR do?

Adds component-styles to <gl-loading-icon> and a small refactor around the application of the base css class in both the component and it's spec.

Note: The added styles are a copy of the existing styles from the CE and haven't been changed to follow the new approach of mixing in utility classes. This will be done in a follow-up, once the sass-helpers are in place: !658 (merged)