[TO_DISCUSS] Publish manuals to the Publishing API

The Publishing API will eventually replace the Content API.

Before merging: 
- [ ] publish manual sections to the Publishing API.
- [ ] publish change notes to the Publishing API.
- [ ] augment the Publishing API payload with organisation info (this needs to be looked up against the Organisations API)
I appreciate that this is inconsistent with the other integration tests; however using the `gds-api-adapters` test helpers:

* means having less test code to maintain in the app
* better encapsulates the interaction with the remote system
* is a pattern that's used across most other apps in the stack

It's a personal preference though - if it's really against the patterns for this app, I could probably change it.