R1:b82863d972a5

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…