Explicitly load assets in revision list

Summary:
Rendering method shouldn't load data.
The view probably shouldn't load data either because it is a job for component (object that both loads data and displays them) but we don't have that concept in Phabricator.
This at least improves the architecture a little bit.

Test Plan: /differential/

Reviewers: epriestley

Reviewed By: epriestley

CC: alanh, aran, Korvin

Differential Revision: https://secure.phabricator.com/D3325