Fix #40: FAQ is very wide on mobile

The wide page is caused by the long lines in the <pre> blocks. We fix this by changing all the <pre> blocks to have class "code", if they weren't already. Then adjust the CSS for the "code" class to add a horizontal scrollbar if needed.

We also had to set the width of the box to 70vw. This was experimentally determined to give a reasonably wide area for the code while not making the regular text wider than the screen.