Rework virus status to check the presence of the file in the infected directory

This prevents the flash of the virus found message as the files are being moved
ensure the clen check checks the file root path
Fix infected? to check both incoming and clean files
Simplify virus status checking code
Merge pull request #743 from alphagov/virus-scanning-changes

Rework virus status to check the presence of the file in the infected directory

https://www.pivotaltracker.com/story/show/54840674