Fix the gh-pages demo for Firefox

[ExplodingCabbage]

Two things were broken:

1. Newlines rendered as massive page breaks instead of newlines, because Firefox creates <div> elements within the contenteditable to represent newlines and our CSS was making them height: 100%.
2. We weren't reading the typed content from the editable divs correctly, because you can't just use textContent or innerText for that - see e.g. https://stackoverflow.com/q/64105609/1709587 and https://stephenhaney.com/2020/get-contenteditable-plaintext-with-correct-linebreaks/

Point 1 is easy to fix, though I worry slightly that some browsers might do really eccentric things (and I don't have a Mac to test Safari). Point 2, there are fixes available (see Stephen Haney's post, or https://stackoverflow.com/a/15349136/1709587) but they're kinda hacky and make me nervous.

Fortunately I don't think we need to use either of those hacks nor do anything clever to fix this, because we can simply replace the contenteditable div with a textarea. We never needed any functionality more complicated than what a textarea provides in the first place!

Fixes #575