Use renderer instead of HttpResponse

[amymok]

This makes use of the renderer functionality DRF provided. This eliminates the use of HttpResponse.

Additions:

• Added CSVRenderer, XLSRenderer, and XLSXRenderer

Removals:

• Removed the use of HttpResponse for non-json response

Changes:

• change the query paramter fmt to format so we can make use of the default usage of DRF renderer

Tests:

• You should now be able to get a file if you do:
  • http://localhost:8000/?format=csv
  • http://localhost:8000/?format=xls
  • http://localhost:8000/?format=xlsx

[coveralls]

Coverage remained the same at 100.0% when pulling f21080d on amymok:use-renderer into f2a3503 on cfpb:master.

[sephcoster]

This looks super tidy and should help make sure our returns are properly formatted as new formats appear and get updated. Nice! 👍