Improve Diffusion browse performance for large files

Summary:
When looking at a large file in Diffusion:

• disable highlighting if it's huge and show a note about why;
• pick up a few other optimizations.

Test Plan: Locally, this improves the main render of __phutil_library_map__.php from 3,200ms to 600ms for me, at the cost of syntax highlighting (we can eventually add view options and let users re-enable it).

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14959