Edition updates should have a clean changeset

www.agileplannerapp.com/boards/173808/cards/5960

This was causing "Scheduled editions cannot be edited"
when trying to publish a scheduled edition now.

That's because an unclean changeset means we incorrectly
identify a change as disallowable:

https://github.com/alphagov/govuk_content_models/blob/master/app/models/workflow.rb#L246-L249

2 factors lead to an unclean changeset:

1. in_beta field was unset for certain editions
   which caused the changeset to have `in_beta_was`
   as `nil`, and `in_beta` as false. the migration
   takes care of setting `in_beta` for editions
   where it is unset.

2. activity attributes are inlined with the edition
   form, so that we can change an edition and transition
   it in one form submission. these fields become a part
   of the changeset. this should not be required as these
   attributes are not necessary beyond the `EditionController`.