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
We'll probably need to change this to `BACKDROP_WRITE_URL` and :back:`DROP_READ_URL` in the future (or just serve read and write from the same port!) But, for now, :+1: