Add Storybook test-runner to run accessibility tests

Leverage storybook to automate accessibility tests with the storybook test runner.

This approach would make existing cypress tests redundant, which is probably a good thing because of the following advantages:

• Component stories & accessibility tests will not be out of sync
• The accessibility add-on in storybook & the tests are also in sync which means we won't have issues like some violations detected by the add-on but not by the tests anymore.
• It's also nice that we can see at the story/component level what rules are disabled.

Relates to gitlab-ui#2959 (closed)