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
  operation within `publishing_api_manual` or `publishing_api_section`
  methods thrice in each controller.  
(Thanks to @jennyd for helping with both of the above.)
- Send a Location header on a success response, as well as the body.
This method is being called three times now, and although it's not a lot of overhead it's unnecessary to instantiate `PublishingApiManual` three times identically. Memoizing makes it visually clear to me that this should be constant throughout the request, so I quite like it, but then we do this a lot in Transition so I'm used to seeing it around.