Restore and display icon assets

[tylersticka]

Overview

• Restores our icons from master to a new location.
• Adds an SVGO config that may be used by any libraries using SVGO.
• Updates config to support importing of SVG files as React components, which makes displaying them in Storybook easier.
• Creates an "Icons" doc page that automatically displays any file in the icon folder.

Known issues

• There isn't an icon CSS or Twig component yet. This is simply asset documentation.
• The existing icons have been included "as-is." No modifications were made to them at this time.
• It's unclear if assets/icons is the right place for these or not. We'll likely want to revisit as more assets are included, for example fonts (Variable font assets #469).
• We may want to only apply the React transformation to SVGs in this particular context.
• Whoa is that a Lanyrd icon?

Screenshot

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See #498