Use EditEngine stacked comments in Diffusion
Summary: Ref T10978. Ref T8739. Fixes T10446. Converts Diffusion to modern comment/preview code, like Differential.
Test Plan:
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10978, T10446, T8739
Differential Revision: https://secure.phabricator.com/D17183