Add a "View as..." option to diff dropdowns for selecting between document engines

Summary:
Depends on D20831. Ref T13425. As an escape hatch to get out of future DocumentEngine rendering behavior, provide a "View As.." option.

Now I can break DocumentEngine real bad and no one can complain.

Test Plan: Used "View As" to swap document engines for image files.

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20832