R1:4310c4ed5387

Track a "Done" state on inline comments

Summary:
Ref T1460. This just barely works, but throwing it up in case any of it sounds mechanically crazy before we build integrations/UI/etc.

Specifically, these are the behaviors:

- You can mark your own draft comments as "done" before you submit them. The intent is to let reviewers mark their stuff advisory/minor/not-important before they submit it, to hint to authors that they don't expect the feedback to necessarily be addressed (maybe it's a joke, maybe…