Split individual transaction specs into separate files
Allow users to enter zero document for legalisation service

See Zendesk ticket 145110. Users sometimes fail to pay for the correct
postage, and the Legalisation office need to be able to send them to the
service to just pay for the postage.
Merge pull request #25 from alphagov/allow_zero_document_count

Allow users to enter zero document for legalisation service