Remove `.gl-spinner` class from gitlab in favor of gitlab-ui
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## Context
The following discussion from gitlab-ui!690 should be addressed:
- [ ] @pslaughter started a [discussion](https://gitlab.com/gitlab-org/gitlab-ui/merge_requests/690#note_235874480):
> **issue:** It looks like there's a `gl-spinner` class already in `gitlab`. This might be problematic... I'll create a follow up issue for us to remove that. :thinking:
issue