Missing require for document association marshall
Swap manuals and documents position in navigation
Access control feature test
Fix test authentication and external service stubbing
Only show documents link if user is from CMA

Also redirect the user back to the manuals page and show an error if they
try to visit the documents page directly.
Extract helper method for document permissions
Manuals now have organisation_slug metadata

When creating a manual, the User's organisation_slug is added to the
manual attrs. This is also used to only allow users from the matching
organisation to access their Org's manuals.
Merge pull request #81 from alphagov/access-control

Access control

https://trello.com/c/dUJ5Z6Pv/85-manuals-access-controls-in-specialist-publisher-3