Switch to the new Response structure

Access the database according to the new Response structure.

The `lib/models/Response.js` change is basically a switch to the model code from the API. Instead of the usual pre-save logic, though, we just have a single pre-save middleware that generates an error, since we should not be saving anything through this model.

Filtering and UTFGrid seem to work. Next step is to look at performance, to make sure we're taking best advantage of the database structure.

/cc @hampelm