Colorize lines in blame under DocumentEngine, to show relative age of changes

Summary:
Depends on D19313. Ref T13105. Fixes T13015. We lost the coloration for ages in the switch to Document Engine.

Restore it, and use a wider range of colors to make the information more clear.

Test Plan: Viewed some blame, saw a nice explosion of bright colors. This is a cornerstone of good design.

Maniphest Tasks: T13105, T13015

Differential Revision: https://secure.phabricator.com/D19314