Use latest govuk_content_models

www.agileplannerapp.com/boards/173808/cards/4288

release 12.4.0 that performs link validations
only on edition updates, so that drafts can be
created from published editions with link validation
errors.
Merge pull request #213 from alphagov/draft-published-editions-having-link-validation-errors

Use latest govuk_content_models