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

- Put the `govuk_url` method onto the `publishing_api_[manual|section]`
  models to avoid mixing internal publishing up with external manual
  validation.
- Memoize (make the variable start with `@_` (?)) to avoid calling the
  `publishing_api_manual` or `publishing_api_section` methods thrice in
  each controller.

(Thanks to Jenny for helping with this.)
Send a Location header on a success response, as well as the body

- The tests for the presence of `/guidance/MANUAL_SLUG` come after the
  assertion `assert_publishing_api_put_item`, because it made more sense
  to me to have them after the thing is sent to publishing API, even
  though the test passes with it before that assertion.
No longer specify Content-Type: application/json as a response header

- Using `render json:` specifies Content-Type `application/json` by
  default.
Use `Plek.current.website_root` instead of `Plek.current.find('www')`
Specify a full URL in expected test output, not just a path

- This method (with an environment variable which Plek uses added to
  `spec_helper`) was taken from
  https://github.com/alphagov/feedback/blob/master/spec/spec_helper.rb#L8.
Add documentation to the README about the GOV.UK URL being returned
Merge pull request #41 from alphagov/return_govuk_url_on_success

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