Make upload app talk to write api

 - Change the upload method to send data to performance platform client
   rather than mongo directly
 - Rework the tests to mock the performance platform client rather than
   checking values in mongo
 - Move tests from feature tests to unit tests. The steps used by our
   feature tests would have needed to post to Backdrop (which isn't
   running). We could have mocked out Backdrop in the feature tests but
   we already do this in our unit tests, and this is the right place to
   test this functionality anyway. This was mainly about moving where
   filters are tested for excel and csv uploading
 - Add performanceplatform-client to requirements.txt
Add build to ackrc
Remove NewDataSet import

- We're not using it anymore
Merge pull request #319 from alphagov/use-write-api

Make upload app talk to write api
Merge pull request #320 from alphagov/add-build-to-ackrc

Add build to ackrc
Add test for empty data in PeriodGroupedData

If an empty data array is passed to a query with a duration parameter,
currently we get a Python exception. This fixes that error.
Merge pull request #322 from alphagov/bugfix/add-test-for-empty-data-with-duration-parameter

Add test for empty data in PeriodGroupedData
Add some more context around upload errors

* Add a data_group and data_type field for easier filtering
* Add a backdrop_upload_error field - I would like to add a tag but the
  logstash formatter we use doesnt support this
* Log the exception info as well
Merge pull request #323 from alphagov/better-upload-error-messages

Add some more context around upload errors