Rename spinner component name to loading-spinner

Problem

The storybook item for the spinner is named loading-icon.

1. Any particular reason why this is not named spinner?
2. Any plans to have another loading-icon that is not a spinner?

Proposal

Let's rename the loading-icon to spinner to match the component name in the design system.

Let's rename the spinner to loading-icon to match the component name in the gitlab-ui storybook.

Let's rename the spinner to loading-spinner and update the matching component in gitlab-ui.

See https://design.gitlab.com/components/spinner v. https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-skeleton-loading--default.