Clean up pages when service is deleted

When services were deleted the Page objects were left in the data store. This deletes all pages associated with a listing when the listing is deleted.

See: https://www.agileplannerapp.com/boards/187120/cards/7893

NOTE: The card also mentions Document objects. As the code stands at present no Documents are ever created.  I think deleting associated Documents when a service is deleted is part of the Documents story (or at least a job for once we have Document uploads working).