When a revision changes more than 1,000 files, don't show the changes on the main page

Summary: Depends on D19295. Ref T13110. Degrade the review UX when users try to interact with changes which are too large to receive human review.

Test Plan: Reduced the "very large" limit, browsed some changes, saw various elements degrade.

Maniphest Tasks: T13110

Differential Revision: https://secure.phabricator.com/D19296