R1:794b456530bc

Store "last comment" and "last action" diffs on reviewers

Summary:
Ref T10967. We have a "commented" state to help reviewers get a better sense of who is part of a discussion, and a "last action" state to help distinguish between "accept" and "accepted an older version", for the purposes of sticky accepts and as a UI hint.

Currently, these are first-class states, partly beacuse we were somewhat limited in what we could do with edges. However, a more flexible way to represent them is as flags separate…