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. Essentially nothing else works yet.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17106