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).
I only had to add `:set -XStandaloneDeriving` to `.ghci` to "support" this change (i.e. to make these files loadable in `ghci`).

`:set -XFlexibleInstances` was added similarly at some point that I can't remember, and at the time I didn't realize I should've un-gitignored it.

The rest of the file's content (including this bit) was there already as its generated by the scaffold.

I tried to capture this as "scaffold + customized" in the [commit message](https://github.com/thoughtbot/carnival/commit/1d254e1cfaa7899963c63a3a17ef72afa26f0d58). I can ammend it to be more detailed though.