use marked instead of pagedown

This is an alternative markdown implementation,
which gives us (optionally) a few things:

- GitHub-flavored markdown (triple-backtick, tables, etc.)
- html sanitization

We need to think very carefully about whether we want to define the notebook markdown as GHM,
as opposed to vanilla markdown, but even if we turn of GHM support (I hope we don't),
marked's sanitization may still be valuable.