When rendering "{image ...}" images, check the cache and just render a direct "<img />" tag if possible

Summary: Depends on D19193. Ref T13101. Fixes T4190. Before we render a fancy AJAX placeholder, check if we already have a valid cache for the image. If we do, render a direct <img /> tag. This is a little cleaner and, e.g., avoids flicker in Safari, at least.

Test Plan: Rendered {image ...} rules in remarkup with new and existing URIs.

Maniphest Tasks: T13101, T4190

Differential Revision: https://secure.phabricator.com/D19194