Use CommentEditEngine in PhamePost

Summary:
Ref T9360. Moves PhamePost to CommentEditEngine.

• HTTP Parameters dropdown on New Post goes to 404
• Implement EditEngine Comments

Test Plan: Make Post, Make Comment, Laugh.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9360

Differential Revision: https://secure.phabricator.com/D16222