rPb48a22bf50a3

Make "editing" state persistent for inline comments

Summary:
Ref T13513. This is mostly an infrastructure cleanup change.

In a perfect world, this would be a series of several changes, but they're tightly interconnected and don't have an obvious clean, nontrivial partition (or, at least, I don't see one). Followup changes will exercise this code repeatedly and all of these individual mutations are "obviously good", so I'm not too worried about the breadth of this change.

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Inline comments are stored…