Resolve issues in CSV downloader

* Remove unused import "re"
* Add import of HttpResponse
* Iterate over qs rather than qs.values() -- the latter isn't right for a ValuesQuerySet
* For a ValuesQuerySet, build header using "field_names" property so it won't have a random order
* Replace unknown variable "filename_param" with intended "filename"
* Add "Cache-Control: no-cache" so browsers won't cache returned files
* Fix lint