This sets the width of code blocks to be the width of the page, and
adds a scrollbar for long blocks. Article `c146e768bb41` has some
examples.
I could have also wrapped the codeblocks, but as pointed out by
[~kaki87] this often reduces readability. Hence: scrollbars.
[~kaki87]: https://todo.sr.ht/~edwardloveall/Scribe/6#event-188395