Implement versioned drafts in EditEngine comment forms

Summary:
Ref T9132. Fixes T5031. This approximately implements the plan described in T5031#67988:

When we recieve a preview request, don't write a draft if the form is from a version of the object before the last update the viewer made.

This should fix the race-related (?) zombie draft comments that sometimes show up.

I just added a new object for this stuff to make it easier to do stacked actions (or whatever we end up with) a little later, since I needed to do some schema adjustments anyway.

Test Plan:

• Typed some text.
• Reloaded page.
• Draft stayed there.
• Tried real hard to get it to ghost by submitting stuff in multiple windows and typing a lot and couldn't, although I didn't bother specifically narrowing down the race condition.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5031, T9132

Differential Revision: https://secure.phabricator.com/D14640