[redesign] Improve the new tabs component so that it is accessible

The interaction for the tabs component has been added in https://github.com/ember-learn/ember-styleguide/pull/157 and this work needs to be improved so that it is fully accessible.

For guidance on how to make a tabs component accessible please refer to https://www.w3.org/TR/wai-aria-practices/examples/tabs/tabs-1/tabs.html