Only enable Disqus JS for comments in production.

This was after @techbelly pointed out that some of the links appearing
were for our respective development environments.

If people want to enable Disqus in development, we should set up a
separate shortname as recommended by Disqus [1].

[1]
http://help.disqus.com/customer/portal/articles/1053796-best-practices-for-staging-development-and-preview-sites