feat(avatars): Add new 40px size and adjust other sizes to base-4

[ryanseddon]

• BREAKING CHANGE?

Description

Default size has changed to new 40px size and older sizes have had their names changed

Pre published: https://garden.zendesk.com/react-components/avatars/

Detail

Introduce a new default 40px size avatar and change other sizes to base-4 system.

zendeskgarden/css-components#149

Bump css avatars dep to new sizing version, update sizes and adds new extrasmall size.

This is breaking due to all sizes changing to base-4.

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• 💂‍♂️ includes new unit and snapshot tests
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage remained the same at 94.729% when pulling a52ad6a on ryan/new_avatar_sizes into ac4cc92 on master.