Add API endpoint /api/v1/acn/downloadKml to AccessionController v1:

@RequestMapping(value = "/downloadKml", produces = "application/vnd.google-earth.kml+xml", method = RequestMethod.POST)
@ResponseBody
public void kml(@RequestParam(value="f") String filterCode, HttpServletResponse response) throws IOException {

    // get AccessionFilter from filterCode
    AccessionFilter filter = ...

    final int countFiltered = accessionService.countAccessions(filter);
    LOG.info("Attempting to download KML for {} accessions", countFiltered);
    if (countFiltered > DOWNLOAD_LIMIT) {
        throw new RuntimeException("Refusing to export more than " + DOWNLOAD_LIMIT + " entries");
    }

    response.setContentType("application/vnd.google-earth.kml+xml");
    response.addHeader("Content-Disposition", String.format("attachment; filename=\"genesys-kml-" + filterCode + ".kml\""));

    // Write KML to the stream.
    final OutputStream outputStream = response.getOutputStream();

    try {
        mappingService.filteredKml(filter, outputStream);
        response.flushBuffer();
    } catch (EOFException e) {
        LOG.warn("Download was aborted", e);
    }
}