Summary:
Ref T13105. This breaks about 9,000 features but moves Diffusion to DocumentEngine for rendering. See T13105 for a more complete list of all the broken stuff.

But you can't bake a software without breaking all the features every time you make a change, right?

Test Plan: Viewed various files in Diffusion, used DocumentEngine features like highlighting and rendering engine selection.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Subscribers: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19302