This patch upgrades Storybook to the latest version. Had to do a decent bit of hacking around with our customizations to get them to keep working. Probably the biggest change was we now have to use an [indexer](https://storybook.js.org/docs/api/main-config-indexers) for our markdown stories, and in order to get the information we need we have to do the same transformations that we do in the loader. For that reason I moved a lot of the loader logic out into a utils file. It feels pretty redundant to have the loader and the indexer...but I'm not sure if there's an alternative right now.
Also worth noting - I turned off the auto generated docs in favor of our READMEs, which I added args tables to. We don't have these for all our components yet, so we might want to file bugs to start adding them. If we would prefer to keep those docs pages for now I think I can configure Storybook to keep generating them
Differential Revision: https://phabricator.services.mozilla.com/D197218
2ef49ff271