Use responsive UI elements in Ponder list views

Summary:
Use the new PhabricatorObjectItemListView in Ponder so it works with the new UI. It will also get some features like flags "for free" in the future.

This removes the pager; I'll restore it in the next diff.

Test Plan: Looked at feed.

Reviewers: pieter, starruler

Reviewed By: pieter

CC: aran, chad

Maniphest Tasks: T1644

Differential Revision: https://secure.phabricator.com/D3507