Add storyshots for automated accessibility testing

[stephiescastle]

Description

Our implementation of Storybook in WWW includes Storyshots, which allows us to automate accessibility testing locally and as part of our CI/CD workflow. While automated testing doesn't catch everything, it has been useful on WWW to catch any outliers and regressions.

All of the components ported from WWW to Explorer 1 have been tested via Storyshots, but are no longer being tested as Explorer 1 does not have Storyshots set up. As Explorer 1 becomes our standard for the design system, I think we should add automated accessibility testing to our workflow here as well.

Task list

• Add storyshots configuration for html stories using axe-core (check config in www)
• Add storyshots to our ci/cd checks

[stephiescastle]

Completed in #484. Not using storyshots, but using Storybook's native test-runner.