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
I'd suggest rewording this test.  Something like "should be possible to create a new draft of an invalid edition", as that would better communicate the intent of this feature.

I also wonder whether it's worth setting up the existing edition with invalid markdown links so that this covers the actual situation we're addressing.