Convert user profile images into a standard cache

Summary:
Ref T4103. Ref T10078. This moves profile image caches to new usercache infrastructure.

These dirty automatically based on configuration and User properties, so add some stuff to make that happen.

This reduces the number of queries issued on every page by 1.

Test Plan: Browsed around, changed profile image, viewed as self, viewed as another user, verified no more query to pull this information on every page

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103, T10078

Differential Revision: https://secure.phabricator.com/D16040