Remove yesod-markdown, use markdown package

No Pandoc!

I manually checked that body_html is rendered HTML, but we should also confirm
on staging. The specifics of the markdown rendering aren't critical (IMO) so
any differences moving from Pandoc to this shouldn't be a big deal (again,
IMO).
It's not that there's a bug or anything to link to. As time has passed since we originally scaffolded Carnival, we have upper bounds on some dependencies (specifically shakespeare and hamlet) preventing us from installing yesod-text-markdown. It'll just be a larger (and widely unrelated) effort to update our framework dependencies. I don't think it's worth holding up dropping pandoc when we can do what I've done here just as easily, though I do plan to get us up on "latest" next time I have a Friday -- which, besides Summit, hasn't been and won't be for a while.