Publish manuals, sections and change notes to the Publishing API

The Publishing API will eventually replace the Content API.

- [x] publish manualsto the Publishing API.
- [x] publish manual sections to the Publishing API.
- [x] publish change notes to the Publishing API.
- [x] augment the Publishing API payload with organisation info (this needs to be looked up against the Organisations API)
I can't get it to work with `/api`:

```ruby
irb(main):001:0> require 'gds_api/organisations'
=> true
irb(main):002:0> GdsApi::Organisations.new('https://www.gov.uk/api').organisation('hm-treasury')
=> nil
irb(main):003:0> GdsApi::Organisations.new('https://www.gov.uk').organisation('hm-treasury')
=> #<GdsApi::Response:0x007fc30918fb68 @http_response="{\"id\":\"https://www.gov.uk/api/organisations/hm-treasury\",\"title\":\"HM Treasury\",\"format\":\"Ministerial department\",\"updated_at\":\"2014-01-15T11:43:56+00:00\",\"web_url\":\"https://www.gov.uk/government/organisations/hm-treasury\",\"details\":{\"slug\":\"hm-treasury\",\"abbreviation\":\"HMT\",\"logo_formatted_name\":\"HM Treasury\",\"organisation_brand_colour_class_name\":\"hm-treasury\",\"organisation_logo_type_class_name\":\"single-identity\",\"closed_at\":null,\"govuk_status\":\"live\"},\"parent_organisations\":[],\"child_organisations\":[{\"id\":\"https://www.gov.uk/api/organisations/uk-debt-management-office\",\"web_url\":\"https://www.gov.uk/government/organisations/uk-debt-management-office\"},{\"id\":\"https://www.gov.uk/api/organisations/uk-financial-investments-limited\",\"web_url\":\"https://www.gov.uk/government/organisations/uk-financial-investments-limited\"},{\"id\":\"https://www.gov.uk/api/organisations/office-for-budget-responsibility\",\"web_url\":\"https://www.gov.uk/government/organisations/office-for-budget-responsibility\"},{\"id\":\"https://www.gov.uk/api/organisations/office-of-tax-simplification\",\"web_url\":\"https://www.gov.uk/government/organisations/office-of-tax-simplification\"},{\"id\":\"https://www.gov.uk/api/organisations/royal-mint\",\"web_url\":\"https://www.gov.uk/government/organisations/royal-mint\"},{\"id\":\"https://www.gov.uk/api/organisations/ns-i\",\"web_url\":\"https://www.gov.uk/government/organisations/ns-i\"},{\"id\":\"https://www.gov.uk/api/organisations/royal-mint-advisory-committee\",\"web_url\":\"https://www.gov.uk/government/organisations/royal-mint-advisory-committee\"},{\"id\":\"https://www.gov.uk/api/organisations/the-crown-estate\",\"web_url\":\"https://www.gov.uk/government/organisations/the-crown-estate\"},{\"id\":\"https://www.gov.uk/api/organisations/infrastructure-uk\",\"web_url\":\"https://www.gov.uk/government/organisations/infrastructure-uk\"}],\"_response_info\":{\"status\":\"ok\",\"links\":[{\"href\":\"https://www.gov.uk/api/organisations/hm-treasury\",\"rel\":\"self\"}]}}">
```