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