R1:082b7f95e6e6

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…