fix storybooks, move to top level, make them easier to use

Created by: sqs

Maintenance on our storybooks.

• All storybooks are now runnable with a single command at the root directory: yarn storybook. Previously, you needed to run client/browser/ and shared/ storybooks separately. Combining them makes it easier to use, discover, and configure storybooks.
• Storybooks are checked in CI. Previously, they had broken (with a Webpack require.context "critical dependency error" due to a Webpack upgrade, and this was not caught because they weren't checked in CI). The check just consists of a smoke test (basically, does Webpack report any errors?).
• The issue that broke storybooks mentioned above was fixed.
• Upgraded @storybook/* to v5.
• Storybooks no longer all import all the global styles. Each *.story.tsx file should import only the necessary CSS. This makes it easier to test shared components with accidentally relying on web app styles or forgetting that style/class props will be necessary.
• Added a CompletionWidget storybook to serve as another example.

Closes #3854 (closed)