chore: Deprecate/Rename Components

[elephantcatdog]

• Renamed UserIcon to Avatar - created copy of 'UserIcon' called 'Avatar' and deprecated 'UserIcon'
  • renamed UserIcon stories and visual tests to Avatar
  • other components now use Avatar instead of UserIcon
  • looks for css from avatar.css and needs the v1 version of css to work correctly
  • added Avatar to core/index.ts
• Renamed UserIconGroup to AvatarGroup - created copy of 'UserIconGroup' called 'AvatarGroup' and deprecated 'UserIconGroup'
  • renamed UserIconGroup stories and visual tests to AvatarGroup
  • added AvatarGroup to core/index.ts
• Renamed Wizard to Stepper - created copy of 'Wizard' called 'Stepper' and deprecated 'Wizard'
  • renamed Wizard stories and visual tests to Stepper
  • moved 'Step.tsx' and 'Step.test.tsx' from the deprecated 'Wizard' folder to the 'Stepper' folder
  • looks for css from stepper.css and needs the v1 version of css to work correctly
  • added Stepper to core/index.ts
• Deprecate HorizontalTab, HorizontalTabs, and VerticalTabs in lieu of Tab and Tabs
  • removed stories for HorizontalTab, HorizontalTabs, and VerticalTabs
  • renamed relevant visual test images and removed uneccessary images
  • updated stories and unit tests to use only Tab and Tabs
• Deprecate Headline, Title, Subheading, Leading, Body, and Small in lieu of Text
  • removed stories for Headline, Title, Subheading, Leading, Body, and Small
  • reimplemented needed stories for Text: Muted, Headline, Title, Subheading, Leading, Body, and Small
  • renamed relevant visual test images and removed uneccessary images
  • added unit test for propagating class names that was in the tests for the deprecated Text types

Note: Git is a bit confused for some files and thinks that Title.test.ts became Avatar/index.ts, but I deleted Title.test.ts and created Avatar/index.ts. Same thing for Small.test.ts & AvatarGroup/index.ts and Leading.test.ts & Stepper/index.ts.

Closes #858

Checklist

• [x] Add meaningful unit tests for your component (verify that all lines are covered)
• [ ] Verify that all existing tests pass
• [x] Add component features demo in Storybook (different stories)
• [ ] Approve test images for new stories
• [ ] Add screenshots of the key elements of the component

[elephantcatdog]

I need to link to v1 css and update/confirm visual tests. Update: this won't be easily possible until the v2 branch is updated to v1