Initial support for comments/append-edits in EditEngine

Summary:
Ref T9132. This just replaces the "Add Comment" form in Paste with a generic flow in EditEngine.

No actual field-awareness or action stacking or anything quite yet, but that will come in a bit. This mildly regresses drafts (which don't seem like a big deal for Pastes). I'll hook those up again in the next diff, but I want to build them in a better way that will work with multiple actions in a generic way, and solve T5031.

Big practical advantage here is that applications don't need copy/pasted preview controllers.

Test Plan:

• Saw previews.
• Added comments.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14637