Simplify cloning-between-Parted-types logic
Simplify cloning-between-Parted-types logic

When cloning an edition:

1. If the origin edition and the target edition are both Parted-type editions,
   we let them handle their own cloning details.

2. If the target edition is of the Parted variety but the origin isn't, we
   handle it differently.

This change extracts the conditional statement for scenario 1 into its own,
hopefully intention-revealing, predicate method.

And simplifies the conditional statement for scenario 2 to its essential check.

Once we're sure that we're not "cloning between parted types", we can safely
check that one or the other (either the origin edition or target edition) is
Parted in the knowledge that the other won't be.

alphagov/govuk_content_models