Refactor: update list classes to carbon classes

[abbeyhrt]

Closes #191

Changed

• imports classes for lists from carbon components
• updates unordered and ordered lists to have carbon class names

New

• creates Li file so list items will have the appropriate class in markdown
  • list items check and see if they have children that are lists and add the class bx--list--nested to the child if so.

Removed

• removes styles we had in place for lists since it's no longer needed

Testing

You can see the changes here: https://gatsby-theme-carbon-git-fork-abbeyhrt-chore-update-list-classes.carbon-design-system.now.sh/components/markdown in the section under list

Here's what it looked like before:

and now:

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[alisonjoseph]

LGTM! Thanks!