Use EditEngine stacked comments in Diffusion

Summary: Ref T10978. Ref T8739. Fixes T10446. Converts Diffusion to modern comment/preview code, like Differential.

Test Plan: {F2342933}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10978, T10446, T8739

Differential Revision: https://secure.phabricator.com/D17183