Rename "DifferentialReviewer" to "DifferentialReviewerProxy"

Summary: Ref T10967. This makes room for a DifferentialReviewer object which can be a real storage table.

Test Plan: Grepped for DifferentialReviewer, browsed Differential.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10967

Differential Revision: https://secure.phabricator.com/D17041