Restructure Response objects

Turn Response objects into parent documents. Subdocuments represent individual entries from the field.

The API should be basically unchanged at this point. Paged queries (with `startIndex` and `count`) will have unintuitive behavior, because they might return more than `count` objects. That's because we request a page of `Response` documents and split the (potentially multiple) entries into separate objects that are returned to the client. We make little to no use of this feature right now, and this shouldn't be a concern later, since we will modify the API in the following ways:
+ Support requests for the parent documents, returned as GeoJSON, so that the number of features returned is exactly `count`
+ Indicate the correct paging parameters to use for the next chunk of data through a response header
+ Support requests for entries after some time. Attempts to get all of the entries can use that feature to "tail" the database, and it will be a consistent way of getting all entries in chunks.

To migrate an existing `responseCollection` to a new `responses` collection:
```bash
$ heroku run node maintenance/migrate-single-response.js -a APP-IN-QUESTION
```