Make rights only migration idempotent

[raphj]

Pages shall only be updated if the rights from the confluence package are not already there. It should not remove existing rights objects only to create the same ones.

It should also probably not affect pages that were modified since the migration (assuming rights were fixed manually). This maybe should be an option.

We may also want the right only migration to avoid creating a revision, or at least we would probably want to keep the dates and authors unaffected.