[RFC] Improved Web UI Flow View

I've been thinking how we can improve the mitmweb flow view UX. In particular, I'd like to address the following points:

1. Ability to view request + response at the same time. This is very useful for replay. This likely means we need add a vertical layout (as opposed to the current horizontal one). We can likely make this configurable without too much effort.
2. Make interactive contentviews available in mitmweb.
3. A bag of quality-of-life improvements, such as colored URL parts.

Here's a quick mockup with some of the ideas:

I'm not a UX designer, so critical feedback and additional suggestions are highly welcome! If you have suggestions for improving mitmweb, please comment below.

Potential Improvements

Small and self-contained (great for new contributors)

• Show reason phrase for HTTP/2 and /3. Not everyone knows status code by heart. Implementation-wise, write a script in ./web/gen that converts https://github.com/mitmproxy/mitmproxy/blob/main/mitmproxy/net/http/status_codes.py to TypeScript. Ideally we add some indicator in the UI that this is not transmitted for h2/h3.
• Colorize response status line background by status code.
• Highlight different URL parts in the request line. This approach from StackOverflow here looks reasonable.
• Add a dedicated "Query" pane for query editing.

Major refactors

• New Layout with extra pane for response. Split existing request/response panes into headers and body?
• Upgrade to the latest CodeMirror (#7709)
• Integrate interactive contentviews.