Refine button links - add icon examples

[anselmbradford]

<button> and <a> tags can both use a-btn classes. This allows making a link that looks like a button and vice versa.

Changes

• Add button link examples that have icons.
• Properly position underline in links with icons that are using a-btn classes.
• Refactor button link SCSS to use mixin.

Testing

1. Visit button and links pages in the PR previews and see the button link and destructive link
   https://deploy-preview-2106--cfpb-design-system.netlify.app/design-system/components/buttons
   https://deploy-preview-2106--cfpb-design-system.netlify.app/design-system/components/links

Screenshots

[netlify]

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