Adds Storybook test cases for scene-ui.js

[DougReeder]

What?

Adds Storybook test cases for scene-ui.js
Also rounds out a few other Storybook stories.

Why?

1. They didn't exist yet, and production code was changed to use the Web Share API.
2. The other stories didn't fully test what they were supposed to.

How to test

1. Run npm run storybook
2. Surf to http://localhost:6006/?path=/story/scene-ui--base
3. Examine each of the scene-ui stories (Show unavailable has white text on a white background, as Storybook could use better style configuration).
4. Navigate to the ChatSidebar — Base story
5. Vary the window width, to see how line breaking behaves.
6. Navigate to Tip — Tooltips
7. In the Controls section at the bottom, select each test case and inspect the UI.

Limitations

None of the Mobile test cases for Tooltip worked before, and they still don't. The UI in the actual app is fine. It's not clear why Storybook gives bad results here.

Alternatives considered

If the Mobile test cases for Tooltip were included, it would be natural for a dev to conclude the actual UI is wrong. That would waste dev time on something that isn't broken in the actual app.

Open questions

How could Storybook be better configured, or the Tooltip story better written?

Additional details or related context

This depends on Matt's Storybook fixes. #6563