Add somewhat better blockquote styling

Quotes are now "quoted" and also have a different background.

I was initially going to make it look like regular replies, with a stripe on the left but it turns out Qt's borders are completely bonkers and it only works if there's a border on all sides. I can't make these corners rounder, because Qt also doesn't support border-radius... But this is a bit better than the weird indented ones we have right now.