Add .ghci

`yesod init` generated most of this file, but it was accidentally ignored by a
global .gitignore. The addition of `:set -XFlexibleInstances` and `:set
-XStandaloneDeriving` are our only customizations.
Remove Yesod.Markdown for Text.Markdown

- This removes the very large dependency text-highlighting-kate, which was
  required by pandoc, which was required by yesod-markdown.
Re-freeze without pandoc