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:

• Made comments and added subscribers and projects.
• Read through the old code to look for missing features and tried to document them all.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14663