Stop duplicate validations being created.

Every time valid_as_draft? or errors_as_draft was called,
apply_any_extra_validations_when_converting_from_imported_to_draft
was adding the validation again, causing duplicate error messages.
Merge pull request #328 from alphagov/duplicate-errors

Stop duplicate validations being created.

https://www.pivotaltracker.com/story/show/45837669