Highlight update time in revision list

Summary:
This is another experiment for reducing reviewers response time.
I stole the idea (and colors) from [[ http://www.reviewboard.org/media/screenshots/2009/02/02/dashboard.png | ReviewBoard ]].
I actually quite like it (except when everything is red) and I can image that people will review just to have better color balance.

The code is not production ready for these reasons:

- We load holidays again and again for each revision. I couldn't cache it to static variable because it could persist multiple requests, right?
- I don't know how to expand height to the whole cell (I'm really bad in CSS).
- CSS rules are probably in wrong file.
- We probably want to use different colors.

This is how it looks:
{F16406}

Test Plan: Displayed revision list.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3190

src/applications/directory/controller/PhabricatorDirectoryMainController.php

@@ -323,6 +323,7 @@ final class PhabricatorDirectoryMainController
         "View Active Revisions \xC2\xBB"));
 
     $revision_view = id(new DifferentialRevisionListView())
+      ->setHighlightAge(true)
       ->setRevisions($active)
       ->setFields(DifferentialRevisionListView::getDefaultFields())
       ->setUser($user);