Get stderr when running clamdscan

Previously, when clamdscan's had an error we haven't had any information about what went wrong. This should address that.
Merge pull request #318 from alphagov/get-clamdscans-stderr

Get stderr when running clamdscan
WIP
This works - mocking the chain of importing client stuff

but it's ugly
Sensible errors for things left to do

- are filters still applied? should they be tested elsewhere
- are auto ids still applied? should they be tested elsewhere
- we need to handle http errors and ignore mongo errors now
Test for filter working

might be some stuff to test on the write api
Fix upload filters and id test

Auto id generation is tested elsewhere. Upload filters are working
correctly.
Refactor simple file post into method
Handle requests exceptions

 - If the upload to backdrop fails, make sure we are given a nice error
page.
 - Add a test that checks what happens if there is a validation error
Refactor mock_post setup
cleanup
WIP
Add performanceplatform-client to requirements.txt
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.
Remove remaining feature tests that are duped

Also remove unnecessary assertion