Unify EditEngine preview behavior; prepare for saving complex drafts

Summary:
Ref T9132. We currently have an old preview/draft behavior and a new actions behavior.

Let the actions behavior do drafts/previews too, so we can eventually throw away the old thing.

This is pretty much just copying the old behavior into the new one, but with a few tweaks. The major change is that we submit all the stacked actions behavior now, so the preview reflects everything the change will do (and, soon, we can save it in the draft in a consistent way).

Also includes one hack-fix that I'll clean up at some point.

Test Plan: Added a bunch of stacked actions and observed meaningful previews.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14672