traducir todas las cosas (i18n demo)

@cookjw 

You can tweak your browser's language settings to see it working (it seems like Accept-Language headers in the request get higher priority than the Django `LANGUAGE_CODE` setting, which is not actually the behavior I'd prefer, because I was imagining a "hypothetical deployment of a non-English Finetooth-based site" rather than a "same instance of Finetooth should cater to speakers of many languages" use case, but whatevs).