Explicitly track inline comment reply threading
Summary:
Ref T1460. Track and store which comments are threaded replies to other comments, vs merely appearing on the same lines.
This doesn't actually write `hasReplies` yet, since that needs to happen when we un-draft comments on submission.
Test Plan: Made inline comments in Differential and Diffusion, including replies. Replies were marked as "Is Reply".
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T1460
Differential Revision: https://secure.phabricator.com/D12017