Remove "Large Changes" documentation and make some minor behavioral improvements

Summary:
Depends on D19296. Ref T13110.

• Remove the "Large Changesets" documentation since we now degrade very large changesets and I don't have any evidence that anyone has ever tried to follow any of the recommendations in this document.
• Remove references to it.
• When an older revision doesn't have denormalized size information on the Revision object itself, don't render a scale element (instead of rendering a bogus one).
• Try to improve terminology consistency around "Large Change" (100-1000 files) vs "Very Large Change" (1000+ files) vs "Enormous Change" (too large to hold in memory).

Test Plan: Viewed revisions; grepped for documentation.

Maniphest Tasks: T13110

Differential Revision: https://secure.phabricator.com/D19298