Add organisation to Publishing API payload

Using an array means that the format (which will be shared with specialist-publisher and then used by manuals-frontend) can accommodate multiple publishing organisations. This isn't currently a requirement, but Specialist Publisher will need it at some point and doing it this way reduces churn in something defined across multiple apps. 

At some point, Publishing API/Content Store will support tagging documents with organisations. At that point, this could be swapped out for that.

/cc @benilovj  @evilstreak
would `merge!` be more appropriate here?