Add key method on CorporateInformationPageType.

Used in RegisterableEdition.
Slightly rationalise path helpers.
Fix Cucumber tests for CIPs.

* Translation and attachments work as normal Editions
* Add force-publish step after saving draft CIPs (and make user a 
  GDS admin so that force-publish is available).
Remove unnecessary conditionals in CIP form.
Simplify overriding document slug for CIP.

Reword explanatory comment.