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 used [here](https://github.com/thoughtbot/carnival/blob/pb-markdown/Model.hs#L47). Really odd that `Text.Markdown` doesn't export the accessor. We could inline a lambda, if it lets us drop the `TL` import too.

```haskell
, "body"         .= ((\Markdown t -> t) $ commentBody c)
```