When a revision has more than 7 reviewers, render only the first 7 in the list view

Summary:
See PHI489. Ref T13110. At least for now, this just shows "..." at the end since you can click the revision to see the whole list anyway.

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