I had it in the `CookbookAuthorizer` at first, but moved it here because there are 2 specific things that are being validated:

1. The user doing this owns the cookbook
2. The person that will become the new owner is currently a collaborator on the cookbook

If we do this in the `CookbookAuthorizer` instead of here in the `CookbookCollaboratorAuthorizer` then we have to explicitly check for the second item and that becomes messy.  Doing it here instead implicitly ensures that the second item is true.