Ref T2543. Further consolidates status management into DifferentialRevisionStatus.
One change I'm making here is internally renaming "CLOSED" to "PUBLISHED". The UI will continue to say "Closed", at least for now, but this should make the code more clear because we care about "is closed, exactly" vs "is any closed status (closed, abandoned, sometimes accepted)". This distinction is more obvious as isClosed() vs isPublished() than, e.g., isClosedWithExactlyTheClosedStatus() or something. I think "Published" is generally more clear, too, and more consistent with modern language (e.g., "pre-publish review" replacing "pre-commit review" to make it more clear what we mean in Git/Mercurial).
I've removed the IN_PREPARATION status since this was just earlier groundwork for "Draft" and not actually used, and under the newer plan I'm trying to just abandon ArcanistDifferentialRevisionStatus entirely (or, at least, substantially).