Attachment virus scanning status

Displays the virus scanning state using the file system and existence of files in certain folders, there is no live update so a page refresh is needed to see the file change state.

We hook into Edition::Publishing to block the publishing of a document, like we do with change notes if all attachments have yet to pass the virus checker. As an aside, I stated to implement the block as a guard on the workflow, which did not seem to block the document from being published - added a story to investigate current guards are holding.

Also display the file count in the tab, as a badge, because I was fed up of randomly opening the tab to see if they had any attachments.

Fixes a ruby 2.0 deprecation with assert_raises

https://www.pivotaltracker.com/story/show/39204833