Serving facts from the Fact Cave

Given data exists in the Fact Cave, enable users to read the data when it's referenced with from a URL.

The Fact Cave API allows accessing stored data through a human-friendly method (the slug) rather than a database key/id.

The response shows the value, the description, and the referenced slug name. The response somewhat matches the structure that govuk_content_api responses take, but as a lot of the glue and URL helpers haven't been put in place yet this is just MVP.