Move content item writes into publishing api client.

In order to be able to separate the publishing part of the API from
content-store in future, a 'publishing-api' vhost alias has been created
for it. This splits the api client for it into two to separate these
concerns.

This will be a major version bump due to the breaking nature of this change.
I totally agree with the consistency, but the mis-capitalisation makes me squirm.