HTML: Use monospace font instead of <pre> block.

[switham]

Right now, the entire notebook page is a single <pre> block. That makes it awkward to put non-monospaced stuff inside it. I think it would work (and better) to just use a monospaced font. That would mean inserting <br> at the end of every code and output line, but I there's already a level of filtering going on (HTML escaping and LF to CRLF).