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.
Allow override of worldwide office access info

Individual offices can be given their own custom access and opening
times information. This is displayed ahead of the organisation's
default information if set. The singular
AccessAndOpeningTimesController has been simplified by combining the
new and edit actions as they are practically the same.