Skip validations while duplicating editions

www.agileplannerapp.com/boards/173808/cards/4296

we have added a new validation for checking invalid links in editions. currently, editors are unable to create drafts from published editions having invalid links because this validation fails.

this proposed change will skip validations while duplicating editions. if there are existing published editions which are deemed invalid because of new validation rules, then editors should be able to create a new edition and rectify those validation errors.

govuk_content_models was updated to remove unnecessary edition save calls which caused validations to happen multiple times when an edition was duplicated, which was needed to support this change:

https://github.com/alphagov/govuk_content_models/pull/187
https://github.com/alphagov/govuk_content_models/pull/186
@alext: fair points.  https://github.com/alphagov/publisher/commit/ec426adf49fb9b3a8d95c3c30cf7186c60d9ee48 makes it better.