The big cahuna
Serve attachment thumbnails

AttachmentController#show needs to handle requests for attachments'
thumbnails. We broke this behaviour in a previous commit.
Tidy up upload access integration tests
Renamed clean_uploads_path to clean_uploads_root
QuarantineFileStorageSimulator no longer needed

Uploaded file storage now behaves the same in all environments, so
this magic is not necessary.
Empty clean_uploads_root at start of each feature
Add Whitehall.incoming_uploads_root
Move config out of CarrierWave and into Whitehall

Now both incoming and clean are configured in the same place and the
configuration is not coupled to carrierwave itself.
Remove unnecessary expand_path call from helper
test-quarantine is no longer used
Use a proper file lock for quarantine testing
Update test cleanup task to reflect new behaviour
Add addtional signposting of monkey-patch
Rake task to fake virus scan in development

Simply copies all incoming files to the clean folder, which is good
enough to simulate a successful virus scan.