Add major change fields to edition workflow
Simplify summary field styles

* Use input classes rather than grid wrappers
* Avoid extra wrapping elements by applying emphasis class to an
existing wrapper
* Use variables for padding and border radius
Apply emphasised field class to beta checkbox
Ensure public_updated_at timestamp is correct

This should be the time of the last major update in the edition series
or the time of the first publish.
Merge pull request #297 from alphagov/major_change_workflow

Major / minor change workflow