* Puts pagination previous/next links in the HTTP header
* Allows the returned field list (for CSV only) to be limited by the
  'fields' query string parameter.
* Flattens CSV list fields into a comma-delimited string, instead of showing list.0,list.1..
* Upgrades djangorestframework-csv to maintain fields/heading sort order in CSV.
* Avoids duplicate-code warnings by moving the CSV response validation tests to
  a new analytics_data_api.v0.tests.views.VerifyCsvResponseMixin