Generate a Sitemap

I tried adding a sitemap in #12, but found that in order to have one automatically generated and hosted on Heroku, we would need to host the sitemap externally, sine Heroku doesn't allow you to modify its project directory after deployment. That would require us to get S3 up and running again, and we just don't feel like doing that at the moment.

To actually add the sitemap generation code, just add back what we removed in aa83c3a723d647bc4e422138f7aac438ad04a2da.