add support for "post-discussion summary"?

[dants]

Similarly to how an "author response" is a first-class citizen in HotCRP rather than a regular comment (allowing one to, e.g., search for submissions that "has:Rebuttalresponse"), it would be very helpful and productive to support a "post-discussion summary", which is frequently used for providing: (i) authors of accepted and major-revision papers with a list of requirements for the camera-ready, and (ii) authors of rejected submissions with an account of what happened and some advice regarding how to move forward.

Commonly, the lead reviewer drafts this summary, and the other reviewers okay it or make revision suggestions, until the summary reflects a consensus. It would therefore be helpful if HotCRP additionally supports the concept of a "finalized post-discussion summary", which indicates that the lead incorporated the feedback from other reviewers and feels the summary is ready to be viewed by the authors.

Currently, we manage the "post-discussion summary" in various ad hoc ways that are less convenient.