Bulk upload works on create too

* BulkUpload::ZipFileToAttachments now works with a blank @edition
* EditionController#build_edition (the before_filter for create that uses the params to build an edition instance) is overridden by EditionController::Attachments to extract_attachments_from_zip_file first (this method remains a before_filter for update actions).
* Failure in EditionController#create will also call extract_edition_information_from_errors (just like EditionController#update does).
Extracting params for bulk upload zip file doesn't render on error

Instead of setting a single error on the edition and re-rendering the form when there is an error with the bulk upload zip file, we instead set a param that forces the edition to fail validation.  This means we don't need the @edition instance to pre-exist before we can safely call EdtionsController::Attachments#extract_attachments_from_zip_file.  As per previous commit it will cope with a nil @edition on success and it won't try to use it on failure, leaving it up to params to do the job.
Merge pull request #172 from alphagov/fix-bulk-upload-on-create

Fix bulk upload on create