This eliminates some duplicate logic, for example the subtle visibility
check in ChangeInserter. The richer information returned by
AddReviewersOp.Result allows us to include reviewers/CCs by email in the
email addresses. This doesn't have any effect yet, since the public
interface to ChangeInserter only allows setting reviewers by account ID,
not by email, but from here that should be an easy feature to add.
Adding reusable support for batching together multiple ReviewerAdditions
unfortunately creates some additional complexity in this interface.
Leave a TODO with some thoughts about how to clean this up, but don't
tackle those yet.
Change-Id: I49ebc841d2ded38460a254ec9fc57c4a5bebd9b8
674531d80a