Take percy snapshots of storybooks

Created by: felixfbecker

Closes #9565 (closed) Closes #10631 (closed)

Also simplifies our Storybook config (Storybook has new recommended config approaches since 5.3).

The coverage and uploading to Percy is separate, because Percy doesn't actually run our storybook in CI. It just gets all static HTML and uploads it to Percy to do the screenshotting in the cloud. We therefor track the coverage by loading each story in Puppeteer in a separate job.

This fixes one important bug that took a long time to find - our Puppeteer coverage reporter overwrote coverage files from previous tests. This must have been a problem for e2e coverage too.

Check out the Percy report in the GitHub status checks below, and the coverage report in the Codecov comment below.