Improve layout of code blocks

[andymatuschak]

Lines probably shouldn't wrap so much with code blocks. Probably this means that code blocks should use fairly different text metrics than the rest of the prompt.

To reproduce

1. Run: cd packages/ui; yarn storybook
2. Visit http://localhost:6006/?path=/story/card--basic
3. Press "A" to open the addons view if necessary, then enter the following into the prompt field:

{"promptType":"qaPrompt","question":{"contents":"What's wrong with this implementation of `sum`?\n\n```\nfunction sum(list: number[]): number {\n    return list.reduce((number, total) => number + total)\n}\n```","attachments":[]},"answer":{"contents":"The arguments to `reduce` are reversed.","attachments":[]}}