Implement PhabricatorMarkupInterface in Diffusion/Audit comments

Summary: Followup to D3804. Makes Diffusion main comments (not just inlines) render properly with the modern markup pipeline.

Test Plan: Created previews and inline previews. Edited inlines. Saved comment, viewed comment. Verified caches were read and written using "Services" tab.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3805