R1:c05306d746d7

Move Differential to EditEngine comments

Summary:
Ref T11114. This is a transitional change that breaks a bunch of stuff. I'll hold it until I've restored features.

This stuff works:

- Commenting.
- Subscribers/tags/reviewers.
- Pinning.
- Drafts.

This stuff does not work yet:

- Preview of inline comments.
- Probably submitting inlines, whatsoever.
- Comment-area warnings like "There are failing tests."
- All meaningful actions (accept, reject, etc).

Test Plan: Commented on a revision.