Ref T2543. This doesn't stand alone since mail still goes out normally, but gets this piece working: new revisions start as "Draft", then after updates if there are no builds they go into "Needs Review".
This should work in general because builds update revisions when they complete, to publish a "Harbormaster finished build yada yada" transaction. So either we'll un-draft immediately, or un-draft after the last build finishes.
I'll hold this until the mail and some other stuff (like UI hints) are in slightly better shape since I think it's probably too rough on its own.