Allow listing with "too many" completed pages to submit

Somehow one supplier's listing has been saved to the datastore with two copies of the same page of responses. By changing the "all pages have been completed" method to return true if there are *more* pages than expected as well as if there are the right number then the user will be able to mark their listing as completed.
Prevent listings with "too many" completed pages happening again

Somehow one supplier's listing has been saved to the datastore with two copies of the same page of responses. If this happens in future this change will remove the duplicate page and ensure there is only ever one 'completedPage' saved per page of questions.
Merge pull request #226 from alphagov/kk_allow_duplicate_page_listing_to_submit

Allow listing with duplicate page to submit