Return the GOV.UK URL to the content in the success JSON response

- Thanks to Jake for debugging the JSON parsing error that was related
  to gds-api-adapters being lazy and not returning a request body (we
  needed `base_path`):
  - Get the `base_path` attribute as we do in manual and section models,
    and to avoid clutter, put this into its own method in each model:
    calling `PublishingApiSection.base_path(@manual_slug, @section_id)`
    for example - now we can call `manual.base_path` or
    `section.base_path` with that line (adapted for manuals or sections)
    inside it.
  - Add to the tests to check that the response body contains the manual
    `/guidance/MANUAL_SLUG` path.