Remove slug pre-registration before publishing

* Wait until a document or manual is published
  before creating an artefact with Panopticon.
* Remove PanopticonMapping instead perform a GET
  request to check existence and either PUT or POST
  using slug.

NOTE:
* All panopticon communication is now via slug
* Panopticon PUT is only used for publishing a
  second version, withdrawal and re-publish.
Remove cruft from document repository spec
Cuke refactor - What even is a mapping?
GdsApiProxy supports not found
More stringent test for publishing second editions
Merge pull request #210 from alphagov/change-panopticon-integration

Change panopticon integration