Show responsiveness' in differential stats

Summary: For "accept" and "reject" action, find the time between the action and last time the revision was updated by a new diff. Show it as the responsiveness row.

Test Plan: view it for a couple of engineers.

Reviewers: epriestley, vii

Reviewed By: epriestley

CC: vrana, nh, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2970