Add "Export Data" link

Per the [Trello card](https://trello.com/c/agCSJ0al/31-instead-of-hiding-showing-columns-export-full-data-spreadsheet-of-results), add an "Export Data" link (styled as a button) to the results table caption, top-right corner. The link `href` gets formatted as:

```
/api/rates/?format=csv&other_params...
```

Where `other_params...` includes all of the form field values passed along to the JSON endpoint.

@kaitlin: The CSV endpoint should probably send a `Content-Disposition: attachment; filename=hourglass-rates.csv` header (for instance) so that browsers download it rather than replacing the web page with raw CSV text.