Add A11y testing to storybook config

[panzacoder]

Add the a11y testing plugin to storybook to ensure a11y standard adherenence.

https://storybook.js.org/addons/@storybook/addon-a11y/

AC:

• a11y tests displaying on all stories.
• any small a11y issues resolved (<15m / story)
• any larger a11y issues documented as tickets (>15m / story)