Support sitemap generation

:construction_worker:

Adding support for sitemap generation as part of https://trello.com/c/e68KlAKz

I added the [sitemap_generator](https://github.com/kjvarga/sitemap_generator) gem to the project, which seems to do what we want really well. I have added and configured the gem, but I have not yet added anything related to running the rake tasks for refreshing the sitemap on deploy or with cron.