Add expand button to code diffs

[halfer]

On another ticket, there was a suggestion to increase the width of the side-by-side diffs, which would make diffs much wider than the body text.

I wonder whether that might look out of place, given that diffs are deliberately the same width as the text content presently. There's a couple of alternatives:

1. Add a JS toggle switch on the page that renders the diffs in wide format
2. Same as above, but toggles the width of the text content too

I think I prefer the first item presently.