Add rake task to repair attachment filenames

The rake task leverages DataHygiene::DocumentAttachmentRepairer to
repair any documents that have attachments with double (or more!)
extensions. It employs a different strategy, depending on the state
of the document (i.e. draft, published or published with a new draft).
Merge pull request #650 from alphagov/repair-attachments

Add rake task to repair attachment filenames

https://www.pivotaltracker.com/story/show/48166823