Rejig routing for admin worldwide offices

Defining the resource as :offices was causing me no end of trouble
because the url helpers that are generated break with the standard
rails conventions. Trying to nest a polymorphic singular resource
under it quickly turned into a nightmare, mainly because you can no
longer rely on rails to just do the right thing when it automatically
generates urls in views and for forms. Using the path: option achieves
the same effect (a neater url), but keeps the routing conventional.
Give worldwide offices their own frontend page

WorldwideOffices now have a slug, scoped to their organisation. The
new page is where the access and opening times information is
displayed. Note that we only link to this page if access information
is actually available.
Failing feature for add custom access info
Make back link go back to whence it came
Correctly nest routing for access info
No need for new action on access info resource

Because it's singular, we can just use the edit action, which
simplifies the routing and url helpers
Correct test typo
Rename WorldwideOffice#access_and_opening_times

We want to create an association with the same name so we rework the
method to simply return the body text. This method can now be made
to return the default or the custom text, depending on what is
available.
WorldwideOffices can have custom access info

Take advantage of the polymorphic controller to allow worldwide
offices to have custom access and opening times information that
overrides the default set on the organisation.