Allow archiving siblings with validation errors

https://govuk.zendesk.com/agent/#/tickets/780846
www.agileplannerapp.com/boards/173808/cards/5005

1. extracting a method to perform state machine
   transitions bypassing validations
2. moving all comments in one place
3. removing unnecessary `save!` calls in tests.
Merge pull request #195 from alphagov/archive-editions-with-validation-errors

Allow archiving siblings with validation errors