As promised, here's a cleanup of block.tsx and related files!

More info:

1. I did a refactor of the code into several React components, rather than 1 component plus several bespoke functions. This makes better use of React's DOM diffing, React keys, and so on. Plus it's easier to read.
2. For performance and code cleanliness, I made it so the autosizer in only used when we have a vertical Block, because that's the only block that holds actual Elements (which need to know their widths). Nothing else needs to know their widths now.
3. I fixed a bug where there was a random parenthesis lying around in ThemedApp.tsx
4. I cleaned up the way we added vertical padding between consecutive elements in an app. This vertical space used to live all over the place but now it's been unified to only live at StyledVerticalBlock.

NOTE: The commit history looks messy because this work came from the Cards project Karen was working on. But I have since pulled out all Card-related code, so the actual diff should be pretty clean.