When a revision has more than 7 reviewers, render only the first 7 in the list view
Also remove the older-style external Handle passing in favor of lazy construction via HandlePool.
Test Plan: Viewed revisions, fiddled with the 7 limit, got sensible-seeming "..." behavior.
Maniphest Tasks: T13110
Differential Revision: https://secure.phabricator.com/D19293