Pull file Document Engine rendering out of "Files" application controllers

Summary:
Ref T13105. This separates document rendering from the Controllers which trigger it so it can be reused elsewhere (notably, in Diffusion).

This shouldn't cause any application behavior to change, it just pulls the rendering logic out so it can be reused elsewhere.

Test Plan: Viewed various types of files in Files; toggled rendering, highlighting, and encoding.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19301