Add filesize limits for document rendering engines and support partial/complete rendering

Summary:
Depends on D19238. Ref T13105. Give document engines some reasonable automatic support for degrading gracefully when someone tries to hexdump a 100MB file or similar.

Also, make "Video" sort above "Audio" for files which could be rendered either way.

Test Plan: Viewed audio, video, image, and other files. Adjusted limits and saw full, partial, and fallback/error rendering.

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19239