Migrate organisation About pages to editions.

For non-worldwide organisations, creates the edition from the
`description` (summary) and `about` (body) fields of the org entity.
For worldwide orgs, convert the `summary` field directly but for body
concatenate `description` and `services`, inserting an "Our services" H1
in between (translated as appropriate for non-English versions).

Add new corporate_information_page_type for these about pages.

Remove translation attributes from org/worldwideorg for the migrated
fields.
Front-end changes for organisation About pages.

* Add methods for accessing about page summary/description from org, use those
in organisation show view.
* Moved controller logic and view for standalone about page from
organisation.about to CIP.index, and update route.
* Add scope to Edition and class method to Organisation to find published
CIPs.
Frontend changes for worldwide_org about pages.

* Add methods for accessing about page fields from worldwide_org models.
* The migration concatenated description+services (including previously
hard-coded H1), so remove separate services block from view.
* Tweak CSS to show that H1 in correct style.
Admin changes for about page CIPs.

* Remove organisation summary/description from form views and controllers.
* Remove standalone About controller method, view, route and menu link.
* Add logic to public routes helper to redirect to correct place for About pages
(complicated by the fact that worldwide orgs and sub-orgs show about text on
org page itself).
* Add about_page? method to CIP model (primarily for use in route helper).
Rename "What we do" to "About us".

The content editors feel that this is a better name both in the admin and on the
front end. About page now renders with "About us" as the H1 and a new "What we
do" h2 in front of the summary text.
Fix org/worldwideorg tests after About changes.
Remove unused attrs from translation views/tests.
Fix functional tests after about_pages changes.

The code that specifically tests behaviour of the About page has moved from the
organisations_controller_test to the corporate_information_pages_controller_test
since that page is now the CIP index for the organisation.
Fix cucumber tests after about_pages changes.
Make non-action controller methods private
Refactor document route helper, add more tests.
Remove translated attributes from WorldwideOrg.

Also remove these fields from the JSON produced by the api presenter. This is
consistent with Organisation.
Pending migrations for removing translated fields.
Extract shared CIP code to a Concern.
Tidy up about_us migration and CIP model.
Merge pull request #1476 from alphagov/about_pages

Convert About pages to editons

https://www.pivotaltracker.com/story/show/69907694