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