R1:6bfb101aff20

Replace all Maniphest commenting code with EditEngine commenting code

Summary:
Ref T9132. Like D14659, I'll hold this until after the cut.

This swaps commenting in Maniphest over to EditEngine / stackable actions. New code doesn't have parity yet, although none of the things we're missing should technically be //strictly mandatory//. There's a list inline. I'll restore these in the next diffs.

Briefly -- comments, subscribers and projects work. Status, owners and priority do not yet.

Test Plan…