Completely move to Supermarket

This introduces some breaking changes. First, the defaults are all now pointing at Supermarket. Additionally, the Supermarket worker now just consumes the `universe.json` endpoint instead of downloading thousands of
cookbooks for indexing.

We are currently seeing exceptionally high loads from users indexing the new Supermarket site, so this would be a great patch to merge. It reduces bandwidth and prevents the need to download every cookbook just to parse the metadata.

I cannot get the tests to run locally :frowning: 

/cc @reset